The lateral surface area of ​​the prism is called. Prism base area: from triangular to polygonal

General information about straight prism

The lateral surface of a prism (more precisely, the lateral surface area) is called sum areas of the side faces. The total surface of the prism is equal to the sum of the lateral surface and the areas of the bases.

Theorem 19.1. The lateral surface of a straight prism is equal to the product of the perimeter of the base and the height of the prism, i.e., the length of the side edge.

Proof. The lateral faces of a straight prism are rectangles. The bases of these rectangles are the sides of the polygon lying at the base of the prism, and the heights are equal to the length of the side edges. It follows that side surface prism is equal

S = a 1 l + a 2 l + ... + a n l = pl,

where a 1 and n are the lengths of the base edges, p is the perimeter of the base of the prism, and I is the length of the side edges. The theorem has been proven.

Practical task

Problem (22) . In an inclined prism it is carried out section, perpendicular to the side ribs and intersecting all the side ribs. Find the lateral surface of the prism if the cross-sectional perimeter is equal to p and the side edges are equal to l.

Solution. The plane of the drawn section divides the prism into two parts (Fig. 411). Let us subject one of them to parallel translation, combining the bases of the prism. In this case, we obtain a straight prism, the base of which is the cross-section of the original prism, and the side edges are equal to l. This prism has the same lateral surface as the original one. Thus, the lateral surface of the original prism is equal to pl.

Summary of the covered topic

Now let’s try to summarize the topic we covered about prisms and remember what properties a prism has.

Prism properties

Firstly, a prism has all its bases as equal polygons;
Secondly, the prism has all its side faces are parallelograms;
Thirdly, in such a multifaceted figure as a prism, all lateral edges are equal;

Also, it should be remembered that polyhedra such as prisms can be straight or inclined.

Which prism is called a straight prism?

If the side edge of a prism is located perpendicular to the plane of its base, then such a prism is called a straight one.

It would not be superfluous to recall that the lateral faces of a straight prism are rectangles.

What type of prism is called oblique?

But if the side edge of a prism is not located perpendicular to the plane of its base, then we can safely say that it is an inclined prism.

Which prism is called correct?

If a regular polygon lies at the base of a straight prism, then such a prism is regular.

Now let us remember the properties that a regular prism has.

Properties of a regular prism

First, always reasons correct prism regular polygons serve;
Secondly, if we consider the side faces of a regular prism, they are always equal rectangles;
Thirdly, if you compare the sizes of the side ribs, then in a regular prism they are always equal.
Fourthly, a correct prism is always straight;
Fifthly, if in a regular prism the lateral faces have the shape of squares, then such a figure is usually called a semi-regular polygon.

Prism cross section

Now let's look at the cross section of the prism:

Homework

Now let's try to consolidate the topic we've learned by solving problems.

Let's draw a slant triangular prism, in which the distance between its edges will be equal to: 3 cm, 4 cm and 5 cm, and the lateral surface of this prism will be equal to 60 cm2. Having these parameters, find the side edge of this prism.

Do you know that geometric figures constantly surround us not only in geometry lessons, but also in Everyday life There are objects that resemble one or another geometric figure.

Everyone at home, at school or at work has a computer, system unit which has the shape of a straight prism.

If you pick up a simple pencil, you will see that the main part of the pencil is a prism.

Walking along the central street of the city, we see that under our feet lies a tile that has the shape of a hexagonal prism.

A. V. Pogorelov, Geometry for grades 7-11, Textbook for educational institutions

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Polygons ABCDE and FHKMP lying in parallel planes are called the bases of the prism, the perpendicular OO 1 lowered from any point of the base to the plane of another is called the height of the prism. Parallelograms ABHF, BCKH, etc. are called the lateral faces of the prism, and their sides SC, DM, etc., connecting the corresponding vertices of the bases, are called lateral edges. In a prism, all lateral edges are equal to each other as segments of parallel straight lines enclosed between parallel planes.
A prism is called a straight line ( Fig. 282, b ) or oblique ( Fig.282,c ) depending on whether its side ribs are perpendicular or inclined to the bases. A straight prism has rectangular side faces. The lateral edge can be taken as the height of such a prism.
A right prism is called regular if its bases are regular polygons. In such a prism, all side faces are equal rectangles.
To depict a prism in a complex drawing, you need to know and be able to depict the elements of which it consists (a point, a straight line, a flat figure).
and their image in a complex drawing ( Fig. 283, a - i)

a) Complex drawing of a prism. The base of the prism is located on the projection plane P 1; one of the side faces of the prism is parallel to the projection plane P 2.
b) The lower base of the prism DEF is a flat figure - a regular triangle located in the plane P 1; the side of the triangle DE is parallel to the x-axis 12 - The horizontal projection merges with the given base and, therefore, is equal to its natural size; The frontal projection merges with the x 12 axis and is equal to the side of the base of the prism.
c) The upper base of the ABC prism is a flat figure - a triangle located in a horizontal plane. The horizontal projection merges with the projection of the lower base and covers it, since the prism is straight; frontal projection - straight, parallel to the x 12 axis, at a distance of the height of the prism.
d) The lateral face of the ABED prism is a flat figure - a rectangle lying in the frontal plane. Frontal projection - a rectangle equal to the natural size of the face; horizontal projection is a straight line equal to the side of the base of the prism.
e) and f) The lateral faces of the ACFD and CBEF prisms are flat figures - rectangles lying in horizontal projecting planes located at an angle of 60° to the projection plane P 2. Horizontal projections are straight lines, located to the x 12 axis at an angle of 60°, and are equal to the natural size of the sides of the base of the prism; frontal projections are rectangles whose images are smaller than life-size: two sides of each rectangle are equal to the height of the prism.
g) Edge AD of the prism is a straight line, perpendicular to the projection plane P 1. Horizontal projection - point; frontal - straight, perpendicular to the x 12 axis, equal to the side edge of the prism (prism height).
h) Side AB of the upper base is straight, parallel to planes P 1 and P 2. Horizontal and frontal projections - straight, parallel to the x 12 axis and equal to the side this basis prisms. The frontal projection is spaced from the x-axis 12 at a distance equal to the height of the prism.
i) The vertices of the prism. Point E - the top of the lower base is located on the plane P 1. The horizontal projection coincides with the point itself; frontal - lies on the x 12 axis. Point C - the top of the upper base - is located in space. Horizontal projection has depth; frontal - height equal to the height of this prism.
This implies: When designing any polyhedron, you need to mentally divide it into its component elements and determine the order of their representation, consisting of successive graphic operations. On ( Fig.284 And Fig.285) examples of sequential graphic operations when performing a complex drawing and visual representation (axonometry) of prisms are given.
(Fig.284).

Given:
1. The base is located on the projection plane P 1.
2. Neither side of the base is parallel to the x-axis 12.
I. Complex drawing.
I, a. We design the lower base - a polygon, which, by condition, lies in the plane P1.
I, b. We design the upper base - a polygon equal to the lower base with sides correspondingly parallel to the lower base, spaced from the lower base by the height H of the given prism.
I, c. We design the side edges of the prism - segments located parallel; their horizontal projections are points merging with the projections of the vertices of the bases; frontal - segments (parallel) obtained from connecting with straight lines the projections of the vertices of the bases of the same name. The frontal projections of the ribs, drawn from the projections of vertices B and C of the lower base, are depicted by dashed lines as invisible.
I, g. Given: horizontal projection F 1 of point F on the upper base and frontal projection K 2 of point K on the side face. It is required to determine the locations of their second projections.
For point F. The second (frontal) projection F 2 of point F will coincide with the projection of the upper base, as a point lying in the plane of this base; its place is determined by the vertical communication line.
For point K - The second (horizontal) projection K 1 of point K will coincide with the horizontal projection of the side face, as a point lying in the plane of the face; its place is determined by the vertical communication line.
II. Prism surface development- a flat figure made up of side faces - rectangles, in which two sides are equal to the height of the prism, and the other two are equal to the corresponding sides of the base, and from two bases equal to each other - irregular polygons.
The natural dimensions of the bases and sides of the faces necessary for constructing the development are revealed on the projections; we build on them; On a straight line we sequentially plot the sides AB, BC, CD, DE and EA of the polygon - the bases of the prism, taken from the horizontal projection. On the perpendiculars drawn from points A, B, C, D, E and A, we plot the height H of this prism taken from the frontal projection and draw a straight line through the marks. As a result, we obtain a scan of the side faces of the prism.
If we attach the bases of the prism to this development, we obtain the development full surface prisms. The bases of the prism should be attached to the corresponding side face using the triangulation method.
On the upper base of the prism, using radii R and R 1, we determine the location of point F, and on the side face, using radius R 3 and H 1, we determine point K.
III. A visual representation of a prism in dimetry.
III, a. We depict the lower base of the prism according to the coordinates of points A, B, C, D and E ( Fig.284 I, a).
III, b. We depict the upper base parallel to the lower one, spaced from it by the height H of the prism.
III, c. We depict the side edges by connecting the corresponding vertices of the bases with straight lines. We determine the visible and invisible elements of the prism and outline them with the corresponding lines,
III, d. We determine points F and K on the surface of the prism - Point F - on the upper base is determined using dimensions i and e; point K - on the side face using i 1 and H" .
For an isometric image of the prism and determining the locations of points F and K, the same sequence should be followed.
Fig.285 ).

Given:
1. The base is located on the plane P 1.
2. The side ribs are parallel to the P 2 plane.
3. Neither side of the base is parallel to the x 12 axis
I. Complex drawing.
I, a. We design according to this condition: the lower base is a polygon lying in the plane P1, and the side edge is a segment parallel to the plane P2 and inclined to the plane P1.
I, b. We design the remaining side edges - segments equal and parallel to the first edge SE.
I, c. We design the upper base of the prism as a polygon, equal and parallel to the lower base, and obtain a complex drawing of the prism.
We identify invisible elements on projections. The frontal projection of the edge of the VM and the horizontal projection of the side of the base CD are depicted by dashed lines as invisible.
I, g. Given the frontal projection Q 2 of the point Q on the projection A 2 K 2 F 2 D 2 of the side face; you need to find its horizontal projection. To do this, draw an auxiliary line through point Q 2 in the projection A 2 K 2 F 2 D 2 of the prism face, parallel to the side edges of this face. We find the horizontal projection of the auxiliary line and on it, using a vertical connection line, we determine the location of the desired horizontal projection Q 1 of point Q.
II. Prism surface development.
Having on a horizontal projection natural sizes sides of the base, and on the front - the dimensions of the ribs, it is possible to construct a complete development of the surface of a given prism.
We will roll the prism, rotating it each time around the side edge, then each side face of the prism on the plane will leave a trace (parallelogram) equal to its natural size. We will construct the side scan in the following order:
a) from points A 2, B 2, D 2. . . E 2 (frontal projections of the vertices of the bases) we draw auxiliary straight lines perpendicular to the projections of the ribs;
b) with a radius R (equal to the side of the base CD), we make a notch at the point D on an auxiliary straight line drawn from point D2; by connecting straight points C 2 and D and drawing straight lines parallel to E 2 C 2 and C 2 D, we obtain the side face CEFD;
c) then, by similarly arranging the following side faces, we obtain a development of the side faces of the prism. To obtain a complete development of the surface of this prism, we attach it to the corresponding faces of the base.
III. A visual representation of a prism in isometry.
III, a. We depict the lower base of the prism and the edge CE, using coordinates according to (