## Note on Pyramid

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Solid objects, as shown below are the pyramids.

As we see above, the pyramid is solid with a polygonal base and triangular faces with a common vertex. A line through the vertex to the centre of the base is called the height of the pyramid. Height perpendicular to the base is called right pyramid otherwise, pyramid is an oblique pyramid. A pyramid is regular if it's all lateral faces are a congruent isosceles triangle.

A pyramid whose base is an equilateral triangle is a tetrahedron. In tetrahedron, all the faces are congruent equilateral triangles.

A perpendicular line segment drawn from the vertex to any side of its base is called the slant height for the face consisting that side.

A pyramid is a three-dimensional solid figure in which the base is a polygon of any number of sides, and other faces are triangles that meet at a common point.

$$\therefore \text {Area of triangular face} = \frac {1} {2} base side \times slant \: height$$

The surface area of the pyramid is the total surface area of its all triangular faces together with the base.

### Volume of a pyramid

Let's take a cubical container of side 'a' units. Take a pyramid of a square base with
a side of length 'a' units and height is same to that of the previous cube. Fill up water in cube by a pyramid.

Cube is filled up when the water is poured three times by the pyramid. By the
above experiment, we can say that the volume of the pyramid is one-third of the
volume of cube whose base and height are the same as that of pyramid. That is, if
V be the volume of the pyramid then, $$V = \frac {1} {3} a^3$$

$$\boxed { \therefore V= \frac {1} {3} \times volume \: of \: the \: cube }$$

It can be written as, $$V= \frac {1} {3} a^2 \times a$$. Hence, $$V= \frac {1} {3} \times base \: area \times height$$

#### Alternatively,

Take a cube of side '2a' units. Draw the space diagonal as shown in the figure.

There are six equal pyramids inside the cube, each has a square base of a side 2a units and height is half of the above cube. One of them is shown to the right of the diagram.

Let V be the volume of each pyramid. The total volume of such six pyramids is same as that of the cube. That is,
\begin{align*} 6V &= (2a)^3 \\ or, 6V &= (2a)^2 2a \\ or, V &= \frac {1} {6} (2a)^2 . 2a \\ \therefore V &= \frac {1} {3} (2a)^2 . a \\ \end{align*}

This means volume of each pyramid is equal to the one-third of product of its base area and height.

$$\therefore V =\frac {1} {3} \times base \: area \times height$$

 In the adjoining figure, Volume of solid = Volume of cuboid + volume of pyramid.TSA of solid = base area + CSA of cuboid + LSA of the pyramid.

A pyramid is the three-dimensional solid figure in which the base is a polygon of any number of sides and other faces are a triangle that meets at a common point.

Area of a triangle face = $$\frac{1}{2}$$ base side × slant height

.

### Very Short Questions

\begin{align*} {\text{Area of square base (A)}} &= (16 cm)^2\\&= 256 cm^2\\ \end{align*}

\begin{align*} {\text{Height of pyramid (h)}} &= \sqrt {(17 cm)^2 - (\frac {16}2 cm)^2}\\ &= \sqrt {289 cm^2 - 64 cm^2}\\ &= \sqrt {225 cm}\\ &= 15 cm\\ \end{align*}

\begin{align*} {\text{Volume of pyramid (V)}} &= \frac 13 Ah\\ &= \frac 13× 256 cm^2× 15 cm\\ &= 1280 cm^3_{Ans}\\ \end{align*}

Suppose,

PQ⊥ BC

Here,

a = BC = 12 cm

h = OP = 8 cm

\begin{align*} \therefore l &=\sqrt {(\frac a2)^2 + h^2}\\ &= \sqrt {(6 cm)^2 + (8 cm)^2}\\ &= \sqrt {100 cm^2}\\ &= 10 cm\\ \end{align*}

Hence,

\begin{align*} {\text{Total surface area of given prism}} &= a^2 + 2al\\ &= (12 cm)^2 + 2 × 12 × 10 cm^2\\ &= 144 cm^2 + 240 cm^2\\ &= 384 cm^2_{Ans}\\ \end{align*}

Side of the squared base (a) = 16 cm

Slant height (l) = 10 cm

\begin{align*} \therefore {\text{Total surface area of the pyramid}} &= a^2 + 2al\\ &= (16 cm)^2 + 2 × 16 cm × 10 cm\\ &= 256 cm^2 + 320 cm^2\\ &= 576 cm^2_{Ans} \end{align*}

Volume of pyramid (V) = 578 cm3

Height of pyramid (h) = 6 cm

Area of square base (A) = ?

Side of square base (a) = ?

We have,

V = $$\frac 13$$ Ah

or, 578 = $$\frac 13$$× A× 6

or, 578 = 2A

or, A = $$\frac {578}2$$

∴ A = 289 cm2Ans

Now,

A = a2

or, a = $$\sqrt A$$

or, a = $$\sqrt (289 cm^2)$$

∴ a = 17 cmAns

Let 'l' be the slant height and 'a' be the length of the side of square base of the pyramis.

Then,

Total surface area = a2 + 2al

By question,

Total surface area = 96 cm2

a = 6 cm

So,

96 = 62 + 2× 6× l

or, 96 - 36 = 12l

or, 60 = 12l

or, l = $$\frac {60}{12}$$

∴ l = 5 cmAns

Let 'a' be the side of the base.

Slant height (l) = 13 cm

Here,

Total surface area = a2 + 2al

or, 360 = a2 + 2a× 13

or, a2 + 26a - 360 = 0

or, a2 + 36a - 10a - 360 = 0

or, a(a + 36) - 10(a + 36) = 0

or, (a - 10) (a + 36) = 0

Either,

a - 10 = 0

∴ a = 10

Or,

a + 36 = 0

∴ a = -36

Since, the length of the side is always positive so a = -36 is impossible.

Hence,

a = 10 cm

\begin{align*} \therefore {\text{Perimeter of base}} &= 4a\\ &= 4× 10 cm\\ &= 40 cm_{Ans}\\ \end{align*}

Here,

PR = 2× OP = 2× 5$$\sqrt 2$$ = 10$$\sqrt 2$$ cm

Let 'a' be the length of a side of the square PQRS then:

PR = $$\sqrt 2$$a

or, 10$$\sqrt 2$$ = $$\sqrt 2$$a

∴ a = 10 cm

If l be the slant height of the pyramid, then:

l2 + $$(\frac a2$$)2= AR2

or, l2 + 52 = 132

or, l2 = 132 - 52

or, l = $$\sqrt {169 - 25}$$

∴ l = 12 cm

\begin{align*} {\text{Total surface area}} &= a^2 + 2al\\ &= (10)^2 + 2× 10× 12\\ &= 100 + 240\\ &= 340 cm^2_{Ans}\\ \end{align*}

0%

26 cm2

>28 cm2

36 cm2

75√3 cm2

60√3 cm2

70√3 cm2

65√3 cm2

50 cm3

41cm3

48 cm3

40 cm3

3030 cm3

3072 cm3

3050 cm3

3070 cm3

• ### A pyramid has squared base of side 24 cm and slant height is 13cm.Find the total surface area and the volume.

1222 cm2,970 cm3

1200cm2,960 cm3

1230 cm2,980 cm3

1240 cm2,930 cm3

• ### A pyramid has a squared base of side 18 cm and height is 12 cm.Calculate the total surface area and the volume.

865 cm2,1293 cm3

866 cm2,1294 cm3

864 cm2,1296 cm3

867 cm2,1295cm3

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