12.5 Surface Area of Pyramids

By Tosha Williams, Taylor Clear, and Jessica Strassle

Objectives

• Find lateral areas of regular pyramids• Find surface areas of regular pyramids

Characteristics of Regular Pyramids

• All of the faces, except the base, intersect at one point called the vertex.

• The base is always a polygon.• The faces that intersect at the vertex are called

lateral faces and form triangles. The edges of the lateral faces that have the vertex as an endpoint are called lateral edges.

• The altitude is the segment from the vertex perpendicular to the base.

Definitions to Know

• If the base of a pyramid is a regular polygon and the segment with endpoints that are the center of the base and the vertex is perp. to the base, then the pyramid is called a regular pyramid.

• The height of each lateral face is called the slant height (l) of the pyramid.

Altitude

Base

Lateral Face

Lateral EdgeVertex

Slant Height

Lateral Area of a Regular Pyramid

• Formula L = ½ Pl P is for Perimeter, L is for the lateral area, l is for

slant height.

Example 1:

• The roof of a birdhouse is a regular hexagonal pyramid. The base of the pyramid has sides of 4 in., and the slant height of the roof is 12in. If the roof is made of copper find the amount of copper used for the roof.

L=1/2Pl

=1/2(24)(12)

=144

Lateral area of a regular pyramid

P=24, l=12. (P=24 because The sides of the base measure 4, so the perimeter is 6(4) )

Multiply.

Your TurnFind the Lateral area of the regular pyramid.

Your Turn

• The answer is 58.2 ft squared.

Surface Area of Regular Pyramid

• Formula T= ½ Pl + B T is the surface area of a regular pyramid, B is

the area of the base.

Example 2:Find the surface area of the square pyramid. To find the surface area, first find the

slant height of the pyramid. The slant height is the hypotenuse of a right triangle with legs that are the altitude and a segment with a length that is one-half the side

measure of the base.

c^2 = a^2 + b^2 Pythagorean Theorem

l^2 = 9^2 + 24^2 a=9, b=24, c=l

l= the square root of 657 Simplify.

Now find the surface area of a regular pyramid. The perimeter of the base is 4(18) or 72 meters, and the area of the base is 18^2 or 324 square meters.

T=1/2Pl + B Surface area of a regular pyramid

T=1/2(72) (Square root of 657) + 324 P=72, l = square root of 657, B = 324

T 1246.8 Use a calculator.

Your Turn

Find the surface area of a regular pyramid.

Your Turn

• The answer is 340 cm squared

Example 3:

• Surface area of pentagonal Pyramid.The altitude, slant height, and apothem form a right triangle. Use the Pythagorean

theorem to find the apothem. Let a represent the length of the apothem.

Use Pythagorean Theorem.

(17)^2 = a^2 + 15^2 b=15, c=17

8= a Simplify.

Now find the length of the sides of the base. The central angle of the pentagon measures 360 /5 or 72 degrees. Let x represent the measure of the angle formed by a radius and the apothem. Then, x = 72/2, or 36.Use Trigonometry to find the length of the sides.

Tan 36degrees = .5s/8

8(tan 36degrees)= .5s Multiply each side by 8

16(tan 36degrees)=s Multiply each side by 2

11.6 = s Use a calculator.

s

8

36

Next, find the perimeter and area of the base.

P= 5s = 5(11.6) or 58.

B= ½ Pa = ½ (58)(8) or 232

Finally, find the surface area.

T= ½ Pl + B Surface area of a regular pyramid

= ½ (58)(17) + 232 P= 58, l= 17, B=232

= 726.5 Simplify.

The surface area is approximately 726.5 square inches.

Assignment:

• Page 663.– 7-16, 18,19, 21-23, 27.

Five test questions

• 1) The Slant height of a regular Square pyramid is 10ft and the sides of the base are 4ft, Find the lateral area.(Answer is 80 ft squared)

• 2) The altitude of a regular Square pyramid is 14 and the side lengths are 25. Find the Slant height and the surface area.(Slant height is 18.8 units squared and the Surface area is 1563.4 units squared.)

Five Test questions(Cont.)

• 3) Find the surface area of a regular pentagonal pyramid if the sides are 10 and the slant height is 13. ( The answer is 497.0 units squared)

• 4) Find the surface area of a regular triangular pyramid if the slant height is 3 and the sides are 5.(The answer is 33.3 units squared)

• 5)Find the surface area of a regular square pyramid if the side is 8 and the slant height is 15.(The answer is 304 units squared)