Equations of higher degrees in mathematics. Equations of higher powers

Trifanova Marina Anatolievna
Mathematics teacher, Gymnasium No. 48 (multiprofile)

The triune purpose of the lesson:

Educational:
systematization and generalization of knowledge on solving equations higher degrees.
Developing:
promote development logical thinking, the ability to work independently, the skills of mutual control and self-control, the ability to speak and listen.
Nurturing:
development of the habit of constant employment, education of responsiveness, hard work, accuracy.

Lesson type:

a lesson in the complex application of knowledge, skills and abilities.

Lesson Form:

airing, physical minute, various forms work.

Equipment:

reference notes, task cards, lesson monitoring matrix.

DURING THE CLASSES

I. Organizational moment

1. Communicate the purpose of the lesson to students.
2. Examination homework(Annex 1). Work with reference summary(Appendix 2).

Equations and answers for each are written on the board. Students check their answers and give brief analysis solve each equation or answer the teacher's questions ( frontal survey). Self-control - students give themselves grades and hand over notebooks to the teacher to check for correction of grades or their approval. Grades school written on the blackboard:

“5+” - 6 equations;
“5” - 5 equations;
“4” - 4 equations;
“3” - 3 equations.

Teacher questions for homework:

1 equation

1. What is the change of variables in the equation?
2. What equation is obtained after the change of variables?

2 equation

1. What polynomial divided both sides of the equation?
2. What substitution of variables was obtained?

3 equation

1. What polynomials need to be multiplied to simplify the solution of this equation?

4 equation

1. Name the function f(x).
2. How were the other roots found?

5 equation

1. How many intervals were obtained to solve the equation?

6 equation

1. How could this equation be solved?
2. Which solution is more rational?

II. Group work is the main part of the lesson.

The class is divided into 4 groups. Each group is given a card with theoretical and practical (Appendix 3) questions: "Disassemble the proposed method for solving the equation and explain it using this example."

1. Group work 15 minutes.
2. Examples are written on the board (the board is divided into 4 parts).
3. The group report takes 2-3 minutes.
4. The teacher corrects the reports of the groups and helps in case of difficulty.

Group work continues on cards No. 5 - 8. For each equation, 5 minutes are given for discussion in the group. Then the blackboard has a report on this equation - a brief analysis of the solution. The equation may not be completely solved - it is being finalized at home, but the entire sequence of its solution in the class is discussed.

III. Independent work. Annex 4.

1. Each student receives an individual assignment.
2. The job takes 20 minutes.
3. 5 minutes before the end of the lesson, the teacher gives open answers for each equation.
4. Students change notebooks in a circle and check the answers with a friend. Giving ratings.
5. Notebooks are handed over to the teacher for checking and correcting grades.

IV. Summary of the lesson.

Homework.

Complete the solution of incomplete equations. Prepare for the control cut.

Grading.

The text of the work is placed without images and formulas.
Full version work is available in the "Files of work" tab in PDF format

Introduction

The solution of algebraic equations of higher degrees with one unknown is one of the most difficult and ancient mathematical problems. The most outstanding mathematicians of antiquity dealt with these problems.

Solving equations of the nth degree is an important task for modern mathematics as well. Interest in them is quite large, since these equations are closely related to the search for the roots of equations that are not considered by the school curriculum in mathematics.

Problem: the lack of skills in solving equations of higher degrees in various ways among students prevents them from successfully preparing for the final certification in mathematics and mathematical olympiads, training in a specialized mathematical class.

The above facts determined relevance of our work "Solution of Equations of Higher Degrees".

Possession of the simplest ways of solving equations of the nth degree reduces the time to complete the task, on which the result of the work and the quality of the learning process depend.

Goal of the work: study of known methods for solving equations of higher degrees and identifying the most accessible of them for practical application.

Based on this goal, the following tasks:

To study the literature and Internet resources on this topic;

Get acquainted with the historical facts related to this topic;

Describe various ways to solve equations of higher degrees

compare the degree of difficulty of each of them;

To acquaint classmates with methods for solving equations of higher degrees;

Create a set of equations for the practical application of each of the considered methods.

Object of study- equations of higher degrees with one variable.

Subject of study- ways of solving equations of higher degrees.

Hypothesis: there is no general way and a single algorithm that allows finding solutions to equations of the nth degree in a finite number of steps.

Research methods:

- biblio graphic method(analysis of literature on the research topic);

- classification method;

- method of qualitative analysis.

Theoretical significance research consists in systematizing methods for solving equations of higher degrees and describing their algorithms.

Practical significance- presented material on this topic and development study guide for students on this topic.

1. EQUATIONS OF THE HIGHER POWERS

1.1 The concept of an equation of the nth degree

Definition 1. An equation of the nth degree is an equation of the form

a 0 xⁿ+a 1 x n -1 +a 2 xⁿ - ²+…+a n -1 x+a n = 0, where the coefficients a 0, a 1, a 2…, a n -1, a n - any real numbers, and ,a 0 ≠ 0 .

Polynomial a 0 xⁿ+a 1 x n -1 +a 2 xⁿ - ²+…+a n -1 x+a n is called a polynomial of the nth degree. Coefficients are distinguished by names: a 0 - senior coefficient; a n is a free member.

Definition 2. Solutions or roots for a given equation are all values ​​of the variable X, which turn this equation into a true numerical equality or, for which the polynomial a 0 xⁿ+a 1 x n -1 +a 2 xⁿ - ²+…+a n -1 x+a n goes to zero. Such a variable value X also called the root of a polynomial. To solve an equation means to find all its roots or establish that there are none.

If a 0 = 1, then such an equation is called a reduced integer rational equation n th degree.

For equations of the third and fourth degree, there are Cardano and Ferrari formulas that express the roots of these equations in terms of radicals. It turned out that in practice they are rarely used. Thus, if n ≥ 3, and the coefficients of the polynomial are arbitrary real numbers, then finding the roots of the equation is not an easy task. However, in many special cases this problem is solved to the end. Let's dwell on some of them.

1.2 Historical facts solutions of equations of higher degrees

Already in ancient times, people realized how important it was to learn how to solve algebraic equations. About 4,000 years ago, Babylonian scientists mastered the solution of a quadratic equation and solved systems of two equations, one of which was of the second degree. With the help of equations of higher degrees, various problems of land surveying, architecture and military affairs were solved, many and varied issues of practice and natural science were reduced to them, since the exact language of mathematics makes it possible to simply express facts and relationships that, being stated plain language may seem confusing and complex.

A universal formula for finding the roots of an algebraic equation n-th no degree. Many, of course, came up with the tempting idea to find formulas for any power of n that would express the roots of the equation in terms of its coefficients, that is, would solve the equation in radicals.

Only in the 16th century, Italian mathematicians managed to advance further - to find formulas for n \u003d 3 and n \u003d 4. At the same time, Scipio, Dahl, Ferro and his students Fiori and Tartaglia were engaged in the question of the general solution of equations of the 3rd degree.

In 1545, the book of the Italian mathematician D. Cardano "Great Art, or on the rules of algebra" was published, where, along with other issues of algebra, common ways solutions of cubic equations, as well as a method for solving equations of the 4th degree, discovered by his student L. Ferrari.

A complete exposition of questions related to the solution of equations of the 3rd and 4th degrees was given by F. Viet.

In the 20s of the 19th century, the Norwegian mathematician N. Abel proved that the roots of equations of the fifth degree cannot be expressed through radicals.

During the study, it was found that modern science there are many ways to solve equations of the nth degree.

The result of the search for methods for solving equations of higher degrees that cannot be solved by the methods considered in school curriculum, have become methods based on the application of the Vieta theorem (for equations of degree n>2), Bezout's theorems, Horner's schemes, as well as the Cardano and Ferrari formula for solving cubic and quartic equations.

The paper presents methods for solving equations and their types, which have become a discovery for us. These include - the method of indefinite coefficients, the allocation of the full degree, symmetric equations.

2. SOLUTION OF INTEGRATED EQUATIONS OF HIGHER POWERS WITH INTEGRATED COEFFICIENTS

2.1 Solution of equations of the 3rd degree. Formula D. Cardano

Consider equations of the form x 3 +px+q=0. Let's transform the equation general view to the view: x 3 +px 2 +qx+r=0. Let's write down the sum cube formula; Let's add it to the original equality and replace it with y. We get the equation: y 3 + (q -) (y -) + (r - =0. After transformations, we have: y 2 +py + q=0. Now, let's write the sum cube formula again:

(a+b) 3 =a 3 + 3a 2 b+3ab 2 +b 3 = a 3 +b 3 + 3ab (a + b), replace ( a+b)on x, we get the equation x 3 - 3abx - (a 3 +b 3) = 0. Now it is clear that the original equation is equivalent to the system: and Solving the system, we get:

We have obtained a formula for solving the above equation of the 3rd degree. It bears the name of the Italian mathematician Cardano.

Consider an example. Solve the equation: .

We have R= 15 and q= 124, then using the Cardano formula we calculate the root of the equation

Conclusion: this formula is good, but not suitable for solving all cubic equations. However, it is bulky. Therefore, it is rarely used in practice.

But the one who masters this formula can use it when solving equations of the third degree in the exam.

2.2 Vieta's theorem

From the course of mathematics, we know this theorem for a quadratic equation, but few people know that it is also used to solve equations of higher degrees.

Consider the equation:

factorize the left side of the equation, divide by ≠ 0.

We transform the right side of the equation to the form

; From this it follows that we can write the following equalities into the system:

The formulas derived by Vieta for quadratic equations and demonstrated by us for equations of the 3rd degree are also true for polynomials of higher degrees.

Let's solve the cubic equation:

Conclusion: this way universal and easy enough for students to understand, since Vieta's theorem is familiar to them from the school curriculum for n = 2. At the same time, in order to find the roots of equations using this theorem, it is necessary to have good computational skills.

2.3 Bezout's theorem

This theorem is named after the 18th century French mathematician J. Bezout.

Theorem. If the equation a 0 xⁿ+a 1 x n -1 +a 2 xⁿ - ²+…+a n -1 x+a n = 0, in which all coefficients are integers, and the free term is different from zero, has an integer root, then this root is a divisor of the free term.

Considering that on the left side of the equation the polynomial nth degree, then the theorem has another interpretation.

Theorem. When dividing a polynomial of the nth degree with respect to x into a binomial x-a remainder equals the value divisible at x = a. (letter a can denote any real or imaginary number, i.e. any complex number).

Proof: let f(x) denotes an arbitrary polynomial of the nth degree with respect to the variable x, and let, when it is divided by a binomial ( x-a) happened in private q(x), and in the remainder R. It's obvious that q(x) there will be some polynomial (n - 1)th degree relatively x, and the remainder R will be a constant value, i.e. independent of x.

If the remainder R was a polynomial of the first degree in x, then this would mean that the division was not performed. So, R from x does not depend. By definition of division, we get the identity: f(x)=(x-a)q(x)+R.

Equality is true for any value of x, so it is also true for x=a, we get: f(a)=(a-a)q(a)+R. Symbol f(a) denotes the value of the polynomial f (x) at x=a, q(a) denotes a value q(x) at x=a. Remainder R remained as it was before R from x does not depend. Work ( x-a) q(a) = 0, since the multiplier ( x-a) = 0, and the multiplier q(a) there is a certain number. Therefore, from the equality we get: f(a)=R, h.t.d.

Example 1 Find the remainder of the division of a polynomial x 3 - 3x 2 + 6x- 5 per binomial

x- 2. By the Bezout theorem : R=f(2) = 23-322 + 62 -5=3. Answer: R= 3.

Note that Bézout's theorem is not so important in itself, but because of its consequences. (Annex 1)

Let us dwell on the consideration of some methods of applying Bezout's theorem to the solution practical tasks. It should be noted that when solving equations using the Bezout theorem, it is necessary:

Find all integer divisors of the free term;

Of these divisors, find at least one root of the equation;

Divide the left side of the equation by (Ha);

Write the product of the divisor and the quotient on the left side of the equation;

Solve the resulting equation.

Consider the example of solving the equation x 3 + 4X 2 + x - 6 = 0 .

Solution: find the divisors of the free term ±1 ; ± 2; ± 3; ± 6. Calculate the values ​​for x= 1, 1 3 + 41 2 + 1-6=0. Divide the left side of the equation by ( X- 1). We perform the division with a “corner”, we get:

Conclusion: Bezout's theorem, one of the ways that we consider in our work, is studied in the program of extracurricular activities. It is difficult to understand, because in order to master it, you need to know all the consequences from it, but at the same time, Bezout's theorem is one of the main assistants to students in the exam.

2.4 Horner's scheme

To divide a polynomial by a binomial x-α you can use a special simple trick invented by English mathematicians of the 17th century, later called Horner's scheme. In addition to finding the roots of equations, Horner's scheme makes it easier to calculate their values. To do this, it is necessary to substitute the value of the variable into the polynomial Pn (x)=a 0 xn+a 1 x n-1 +a 2 xⁿ - ²+…++ a n -1 x+a n. (1)

Consider the division of the polynomial (1) by the binomial x-α.

We express the coefficients of the incomplete quotient b 0 xⁿ - ¹+ b 1 xⁿ - ²+ b 2 xⁿ - ³+…+ bn -1 and the remainder r in terms of the coefficients of the polynomial Pn( x) and number α. b 0 =a 0 , b 1 = α b 0 +a 1 , b 2 = α b 1 +a 2 …, bn -1 =

= α bn -2 +a n -1 = α bn -1 +a n .

Calculations according to the Horner scheme are presented in the form of the following table:

Because the r=Pn(α), then α is the root of the equation. In order to check whether α is a multiple root, Horner's scheme can be applied already to the quotient b 0 x+ b 1 x+…+ bn -1 according to the table. If in the column under bn -1 we get 0 again, so α is a multiple root.

Consider an example: solve the equation X 3 + 4X 2 + x - 6 = 0.

Let us apply to the left side of the equation the factorization of the polynomial on the left side of the equation, Horner's scheme.

Solution: find the divisors of the free term ± 1; ±2; ± 3; ± 6.

The coefficients of the quotient are the numbers 1, 5, 6, and the remainder is r = 0.

Means, X 3 + 4X 2 + X - 6 = (X - 1) (X 2 + 5X + 6) = 0.

From here: X- 1 = 0 or X 2 + 5X + 6 = 0.

X = 1, X 1 = -2; X 2 = -3. Answer: 1,- 2, - 3.

Conclusion: thus, on one equation, we have shown the use of two various ways factorizations of polynomials. In our opinion, Horner's scheme is the most practical and economical.

2.5 Solution of equations of the 4th degree. Ferrari Method

Cardano's student Ludovic Ferrari discovered a way to solve a 4th degree equation. The Ferrari method consists of two steps.

Stage I: the equation of the form is represented as a product of two square trinomials; this follows from the fact that the equation is of the 3rd degree and at least one solution.

Stage II: the resulting equations are solved using factorization, however, in order to find the required factorization, one has to solve cubic equations.

The idea is to represent the equations as A 2 =B 2 where A= x 2+s,

B-linear function of x. Then it remains to solve the equations A = ±B.

For clarity, consider the equation: We separate the 4th degree, we get: For any d expression will be a perfect square. Add to both sides of the equation we get

On the left side is a full square, you can pick up d so that the right side of (2) becomes a perfect square. Imagine that we have achieved this. Then our equation looks like this:

Finding the root later will not be difficult. To choose the right d it is necessary that the discriminant of the right side of (3) vanishes, i.e.

So to find d, it is necessary to solve this equation of the 3rd degree. This auxiliary equation is called resolvent.

We can easily find the integer root of the resolvent: d= 1

Substituting the equation into (1), we obtain

Conclusion: the Ferrari method is universal, but complicated and cumbersome. At the same time, if the solution algorithm is clear, then equations of the 4th degree can be solved by this method.

2.6 Method of undetermined coefficients

The success of solving the equation of the 4th degree by the Ferrari method depends on whether we solve the resolvent - the equation of the 3rd degree, which, as we know, is not always possible.

The essence of the method of indefinite coefficients is that the type of factors into which a given polynomial is decomposed is guessed, and the coefficients of these factors (also polynomials) are determined by multiplying the factors and equating the coefficients at the same powers of the variable.

Example: solve the equation:

Suppose that the left side of our equation can be decomposed into two square trinomials with integer coefficients such that the identical equality

It is obvious that the coefficients in front of them must be equal to 1, and the free terms must be equal to one + 1, the other has 1.

The coefficients facing X. Let's denote them by A and and to determine them, we multiply both trinomials on the right side of the equation.

As a result, we get:

Equating the coefficients at the same powers X on the left and right sides of equality (1), we obtain a system for finding and

Solving this system, we will have

So our equation is equivalent to the equation

Solving it, we get the following roots: .

The method of indefinite coefficients is based on the following statements: any polynomial of the fourth degree in the equation can be decomposed into the product of two polynomials of the second degree; two polynomials are identically equal if and only if their coefficients are equal at the same powers X.

2.7 Symmetric equations

Definition. An equation of the form is called symmetric if the first coefficients on the left of the equation are equal to the first coefficients on the right.

We see that the first coefficients on the left are equal to the first coefficients on the right.

If such an equation has an odd degree, then it has a root X= - 1. Next, we can lower the degree of the equation by dividing it by ( x+ 1). It turns out that when dividing the symmetric equation by ( x+ 1) a symmetric equation of even degree is obtained. The proof of the symmetry of the coefficients is presented below. (Appendix 6) Our task is to learn how to solve symmetric equations of even degree.

For example: (1)

We solve equation (1), divide by X 2 (to the middle degree) = 0.

We group the terms with symmetric

) + 3(x+ . Denote at= x+ , let's square both parts, hence = at 2 So 2( at 2 or 2 at 2 + 3 solving the equation, we get at = , at= 3. Next, we return to the replacement x+ = and x+ = 3. We get the equations and The first has no solution, and the second has two roots. Answer:.

Conclusion: this type of equation is not often encountered, but if you come across it, then it can be solved easily and simply without resorting to cumbersome calculations.

2.8 Extraction of the full degree

Consider the equation.

The left side is the cube of the sum (x + 1), i.e.

We extract the root of the third degree from both parts: , then we get

Where is the only root.

RESULTS OF THE STUDY

As a result of the work, we came to the following conclusions:

Thanks to the studied theory, we got acquainted with various methods for solving entire equations of higher degrees;

D. Cardano's formula is difficult to use and gives a high probability of making errors in the calculation;

− the method of L. Ferrari allows to reduce the solution of the equation of the fourth degree to the cubic one;

− Bezout's theorem can be used both for cubic equations and for equations of the fourth degree; it is more understandable and illustrative when applied to solving equations;

Horner's scheme helps to significantly reduce and simplify calculations in solving equations. In addition to finding the roots, Horner's scheme makes it easier to calculate the values ​​of the polynomials on the left side of the equation;

Of particular interest was the solution of equations by the method of indefinite coefficients, the solution of symmetric equations.

During research work it was found that students get acquainted with the simplest methods of solving equations of the highest degree at elective classes in mathematics, starting from the 9th or 10th grade, as well as at special courses of traveling mathematical schools. This fact established as a result of a survey of mathematics teachers at MBOU "Secondary School No. 9" and students showing increased interest in the subject "mathematics".

The most popular methods for solving equations of higher degrees, which are encountered in solving olympiad, competitive problems and as a result of preparing for exams by students, are methods based on the application of Bezout's theorem, Horner's scheme and the introduction of a new variable.

Demonstration of the results of research work, i.e. ways to solve equations that are not studied in the school curriculum in mathematics, interested classmates.

Conclusion

Having studied the educational and scientific literature, Internet resources in youth educational forums

In general, an equation that has a degree higher than 4 cannot be solved in radicals. But sometimes we can still find the roots of the polynomial on the left in the equation of the highest degree, if we represent it as a product of polynomials in a degree of no more than 4. The solution of such equations is based on the decomposition of the polynomial into factors, so we advise you to review this topic before studying this article.

Most often, one has to deal with equations of higher degrees with integer coefficients. In these cases, we can try to find rational roots, and then factor the polynomial so that we can then convert it to an equation of a lower degree, which will be easy to solve. In the framework of this material, we will consider just such examples.

Yandex.RTB R-A-339285-1

Higher degree equations with integer coefficients

All equations of the form a n x n + a n - 1 x n - 1 + . . . + a 1 x + a 0 = 0 , we can reduce to an equation of the same degree by multiplying both sides by a n n - 1 and changing the variable of the form y = a n x:

a n x n + a n - 1 x n - 1 + . . . + a 1 x + a 0 = 0 a n n x n + a n - 1 a n n - 1 x n - 1 + ... + a 1 (a n) n - 1 x + a 0 (a n) n - 1 = 0 y = a n x ⇒ y n + b n - 1 y n - 1 + … + b 1 y + b 0 = 0

The resulting coefficients will also be integers. Thus, we will need to solve the reduced equation of the nth degree with integer coefficients, which has the form x n + a n x n - 1 + ... + a 1 x + a 0 = 0.

We calculate the integer roots of the equation. If the equation has integer roots, you need to look for them among the divisors of the free term a 0. Let's write them down and substitute them into the original equality one by one, checking the result. Once we have obtained an identity and found one of the roots of the equation, we can write it in the form x - x 1 · P n - 1 (x) = 0 . Here x 1 is the root of the equation, and P n - 1 (x) is the quotient of x n + a n x n - 1 + ... + a 1 x + a 0 divided by x - x 1 .

Substitute the remaining divisors in P n - 1 (x) = 0 , starting with x 1 , since the roots can be repeated. After obtaining the identity, the root x 2 is considered found, and the equation can be written as (x - x 1) (x - x 2) P n - 2 (x) \u003d 0. Here P n - 2 (x) will be quotient from dividing P n - 1 (x) by x - x 2 .

We continue to sort through the divisors. Find all integer roots and denote their number as m. After that, the original equation can be represented as x - x 1 x - x 2 · … · x - x m · P n - m (x) = 0 . Here P n - m (x) is a polynomial of n - m -th degree. For calculation it is convenient to use Horner's scheme.

If our original equation has integer coefficients, we cannot end up with fractional roots.

As a result, we got the equation P n - m (x) = 0, the roots of which can be found in any convenient way. They can be irrational or complex.

Let's show on specific example how such a solution scheme is applied.

Example 1

Condition: find the solution of the equation x 4 + x 3 + 2 x 2 - x - 3 = 0 .

Solution

Let's start with finding integer roots.

We have an intercept equal to minus three. It has divisors equal to 1 , - 1 , 3 and - 3 . Let's substitute them into the original equation and see which of them will give identities as a result.

For x equal to one, we get 1 4 + 1 3 + 2 1 2 - 1 - 3 \u003d 0, which means that one will be the root of this equation.

Now let's divide the polynomial x 4 + x 3 + 2 x 2 - x - 3 by (x - 1) into a column:

So x 4 + x 3 + 2 x 2 - x - 3 = x - 1 x 3 + 2 x 2 + 4 x + 3.

1 3 + 2 1 2 + 4 1 + 3 = 10 ≠ 0 (- 1) 3 + 2 (- 1) 2 + 4 - 1 + 3 = 0

We got an identity, which means we found another root of the equation, equal to - 1.

We divide the polynomial x 3 + 2 x 2 + 4 x + 3 by (x + 1) in a column:

We get that

x 4 + x 3 + 2 x 2 - x - 3 = (x - 1) (x 3 + 2 x 2 + 4 x + 3) = = (x - 1) (x + 1) (x 2 + x + 3)

We substitute the next divisor into the equation x 2 + x + 3 = 0, starting from - 1:

1 2 + (- 1) + 3 = 3 ≠ 0 3 2 + 3 + 3 = 15 ≠ 0 (- 3) 2 + (- 3) + 3 = 9 ≠ 0

The resulting equalities will be incorrect, which means that the equation no longer has integer roots.

The remaining roots will be the roots of the expression x 2 + x + 3 .

D \u003d 1 2 - 4 1 3 \u003d - 11< 0

It follows from this that this square trinomial does not have real roots, but there are complex conjugate ones: x = - 1 2 ± i 11 2 .

Let us clarify that instead of dividing into a column, Horner's scheme can be used. This is done like this: after we have determined the first root of the equation, we fill in the table.

In the table of coefficients, we can immediately see the coefficients of the quotient from the division of polynomials, which means x 4 + x 3 + 2 x 2 - x - 3 = x - 1 x 3 + 2 x 2 + 4 x + 3.

After finding the next root, equal to - 1 , we get the following:

Answer: x \u003d - 1, x \u003d 1, x \u003d - 1 2 ± i 11 2.

Example 2

Condition: solve the equation x 4 - x 3 - 5 x 2 + 12 = 0.

Solution

The free member has divisors 1 , - 1 , 2 , - 2 , 3 , - 3 , 4 , - 4 , 6 , - 6 , 12 , - 12 .

Let's check them in order:

1 4 - 1 3 - 5 1 2 + 12 = 7 ≠ 0 (- 1) 4 - (- 1) 3 - 5 (- 1) 2 + 12 = 9 ≠ 0 2 4 2 3 - 5 2 2 + 12 = 0

So x = 2 will be the root of the equation. Divide x 4 - x 3 - 5 x 2 + 12 by x - 2 using Horner's scheme:

As a result, we get x - 2 (x 3 + x 2 - 3 x - 6) = 0 .

2 3 + 2 2 - 3 2 - 6 = 0

So 2 will again be a root. Divide x 3 + x 2 - 3 x - 6 = 0 by x - 2:

As a result, we get (x - 2) 2 (x 2 + 3 x + 3) = 0 .

Checking the remaining divisors does not make sense, since the equality x 2 + 3 x + 3 = 0 is faster and more convenient to solve using the discriminant.

Let's solve the quadratic equation:

x 2 + 3 x + 3 = 0 D = 3 2 - 4 1 3 = - 3< 0

We get a complex conjugate pair of roots: x = - 3 2 ± i 3 2 .

Answer: x = - 3 2 ± i 3 2 .

Example 3

Condition: find the real roots for the equation x 4 + 1 2 x 3 - 5 2 x - 3 = 0.

Solution

x 4 + 1 2 x 3 - 5 2 x - 3 = 0 2 x 4 + x 3 - 5 x - 6 = 0

We perform the multiplication 2 3 of both parts of the equation:

2 x 4 + x 3 - 5 x - 6 = 0 2 4 x 4 + 2 3 x 3 - 20 2 x - 48 = 0

We replace the variables y = 2 x:

2 4 x 4 + 2 3 x 3 - 20 2 x - 48 = 0 y 4 + y 3 - 20 y - 48 = 0

As a result, we got a standard equation of the 4th degree, which can be solved according to the standard scheme. Let's check the divisors, divide and in the end we get that it has 2 real roots y \u003d - 2, y \u003d 3 and two complex ones. We will not present the entire solution here. By virtue of the replacement, the real roots of this equation will be x = y 2 = - 2 2 = - 1 and x = y 2 = 3 2 .

Answer: x 1 \u003d - 1, x 2 \u003d 3 2

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Methods for solving equations: n n n Replacement of the equation h(f(x)) = h(g(x)) by the equation f(x) = g(x) Factorization. Introduction of a new variable. Functional - graphical method. Root selection. Application of Vieta formulas.

Replacing the equation h(f(x)) = h(g(x)) by the equation f(x) = g(x). The method can be applied only when y = h(x) is a monotonic function that takes each of its values ​​once. If the function is nonmonotone, then the loss of roots is possible.

Solve the equation (3 x + 2)²³ = (5 x - 9)²³ y = x ²³ increasing function, so from the equation (3 x + 2)²³ = (5 x - 9)²³ you can go to the equation 3 x + 2 \u003d 5 x - 9, from where we find x \u003d 5.5. Answer: 5.5.

Factorization. The equation f(x)g(x)h(x) = 0 can be replaced by the set of equations f(x) = 0; g(x) = 0; h(x) = 0. Having solved the equations of this set, you need to take those roots that belong to the domain of definition of the original equation, and discard the rest as extraneous.

Solve the equation x³ - 7 x + 6 = 0 Representing the term 7 x as x + 6 x, we get sequentially: x³ - x - 6 x + 6 = 0 x(x² - 1) - 6(x - 1) = 0 x (x - 1)(x + 1) - 6(x - 1) = 0 (x - 1)(x² + x - 6) = 0 Now the problem is reduced to solving a set of equations x - 1 = 0; x² + x - 6 = 0. Answer: 1, 2, - 3.

Introduction of a new variable. If the equation y(x) = 0 can be transformed to the form p(g(x)) = 0, then you need to introduce a new variable u = g(x), solve the equation p(u) = 0, and then solve the set of equations g( x) = u 1; g(x) = u2; … ; g(x) = un , where u 1, u 2, … , un are the roots of the equation p(u) = 0.

Solve the equation A feature of this equation is the equality of the coefficients of its left side, equidistant from its ends. Such equations are called reciprocal. Since 0 is not the root of this equation, dividing by x² gives

Let's introduce a new variable Then We get a quadratic equation So the root y 1 = - 1 can be ignored. We get the Answer: 2, 0, 5.

Solve the equation 6(x² - 4)² + 5(x² - 4)(x² - 7 x +12) + (x² - 7 x + 12)² = 0 This equation can be solved as homogeneous. Divide both sides of the equation by (x² - 7 x +12)² (it is clear that x values ​​such that x² - 7 x +12=0 are not solutions). Now let's denote We Have From Here Answer:

Functional - graphical method. If one of the functions y \u003d f (x), y \u003d g (x) increases, and the other decreases, then the equation f (x) \u003d g (x) either has no roots or has one root.

Solve the equation It is quite obvious that x = 2 is the root of the equation. Let us prove that this is the only root. We transform the equation to the form We notice that the function is increasing, and the function is decreasing. So the equation has only one root. Answer: 2.

Selection of roots n n n Theorem 1: If an integer m is a root of a polynomial with integer coefficients, then the constant term of the polynomial is divisible by m. Theorem 2: The reduced polynomial with integer coefficients has no fractional roots. Theorem 3: – equation with integer Let coefficients. If the number and fraction where p and q are integers is irreducible, is the root of the equation, then p is the divisor of the free term an, and q is the divisor of the coefficient at the highest term a 0.

Bezout's theorem. The remainder when dividing any polynomial by a binomial (x - a) is equal to the value of the divisible polynomial at x = a. Consequences of Bezout's theorem n n n n The difference of identical powers of two numbers is divisible without remainder by the difference of the same numbers; The difference of identical even powers of two numbers is divisible without remainder both by the difference of these numbers and by their sum; The difference of identical odd powers of two numbers is not divisible by the sum of these numbers; The sum of equal powers of two non-numbers is divisible by the difference of these numbers; The sum of identical odd powers of two numbers is divisible without a remainder by the sum of these numbers; The sum of identical even powers of two numbers is not divisible either by the difference of these numbers or by their sum; The polynomial is divisible by the binomial (x - a) if and only if the number a is the root of this polynomial; The number of distinct roots of a non-zero polynomial is no more than its degree.

Solve the equation x³ - 5 x² - x + 21 = 0 The polynomial x³ - 5 x² - x + 21 has integer coefficients. By Theorem 1, its integer roots, if any, are among the divisors of the free term: ± 1, ± 3, ± 7, ± 21. By checking, we make sure that the number 3 is a root. By a corollary of Bezout's theorem, the polynomial is divisible by (x – 3). Thus, x³ - 5 x² - x + 21 \u003d (x - 3) (x² - 2 x - 7). Answer:

Solve the equation 2 x³ - 5 x² - x + 1 = 0 According to Theorem 1, only numbers ± 1 can be integer roots of the equation. Checking shows that these numbers are not roots. Since the equation is not reduced, it can have fractional rational roots. Let's find them. To do this, multiply both sides of the equation by 4: 8 x³ - 20 x² - 4 x + 4 = 0 By substituting 2 x = t, we get t³ - 5 t² - 2 t + 4 = 0. By Terem 2, all rational roots of this reduced equation must be whole. They can be found among the divisors of the constant term: ± 1, ± 2, ± 4. In this case, t \u003d - 1 is suitable. Therefore, the polynomial 2 x³ - 5 x² - x + 1 is divisible by (x + 0, 5 ): 2 x³ - 5 x² - x + 1 \u003d (x + 0, 5) (2 x² - 6 x + 2) Solving the quadratic equation 2 x² - 6 x + 2 \u003d 0, we find the remaining roots: Answer:

Solve the equation 6 x³ + x² - 11 x - 6 = 0 According to Theorem 3, the rational roots of this equation should be sought among the numbers. Substituting them one by one into the equation, we find that they satisfy the equation. They exhaust all the roots of the equation. Answer:

Find the sum of squares of the roots of the equation x³ + 3 x² - 7 x +1 = 0 By the Vieta theorem Note that whence

Specify the method by which each of these equations can be solved. Solve Equations #1, 4, 15, 17.

Answers and instructions: 1. Introduction of a new variable. 2. Functional - graphical method. 3. Replacing the equation h(f(x)) = h(g(x)) by the equation f(x) = g(x). 4. Factorization. 5. Selection of roots. 6 Functionally - graphical method. 7. Application of Vieta formulas. 8. Selection of roots. 9. Replacing the equation h(f(x)) = h(g(x)) by the equation f(x) = g(x). 10. Introduction of a new variable. 11. Factorization. 12. Introduction of a new variable. 13. Selection of roots. 14. Application of Vieta formulas. 15. Functional - graphical method. 16. Factorization. 17. Introduction of a new variable. 18. Factorization.

1. Instruction. Write the equation as 4(x²+17 x+60)(x+16 x+60)=3 x², Divide both sides by x². Enter variable Answer: x 1 = - 8; x 2 \u003d - 7, 5. 4. Indication. Add 6 y and - 6 y to the left side of the equation and write it as (y³ - 2 y²) + (- 3 y² + 6 y) + (- 8 y + 16) = (y - 2)(y² - 3 y - 8). Answer:

14. Instruction. According to Vieta's theorem Since - are integers, then only the numbers - 1, - 2, - 3 can be the roots of the equation. Answer: 15. Answer: - 1. 17. Indication. Divide both sides of the equation by x² and write it as Enter a variable Answer: 1; 15; 2; 3.

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