 # Surface Area of Prisms and Cylinders 10-4 Holt Geometry.

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Surface Area of Prisms and Cylinders 10-4 Holt Geometry

Warm Up Find the perimeter and area of each polygon. 1. a rectangle with base 14 cm and height 9 cm 2. a right triangle with 9 cm and 12 cm legs 3. an equilateral triangle with side length 6 cm

Objectives Learn and apply the formula for the surface area of a prism. Learn and apply the formula for the surface area of a cylinder.

Vocabulary lateral face lateral edge right prism oblique prism
altitude surface area lateral surface axis of a cylinder right cylinder oblique cylinder

Prisms and cylinders have 2 congruent parallel bases.
A lateral face is not a base. The edges of the base are called base edges. A lateral edge is not an edge of a base. The lateral faces of a right prism are all rectangles. An oblique prism has at least one nonrectangular lateral face.

An altitude of a prism or cylinder is a perpendicular segment joining the planes of the bases. The height of a three-dimensional figure is the length of an altitude. Surface area is the total area of all faces and curved surfaces of a three-dimensional figure. The lateral area of a prism is the sum of the areas of the lateral faces.

The net of a right prism can be drawn so that the lateral faces form a rectangle with the same height as the prism. The base of the rectangle is equal to the perimeter of the base of the prism.

The surface area of a right rectangular prism with length ℓ, width w, and height h can be written as
S = 2ℓw + 2wh + 2ℓh.

The surface area formula is only true for right prisms
The surface area formula is only true for right prisms. To find the surface area of an oblique prism, add the areas of the faces. Caution!

Example 1A: Finding Lateral Areas and Surface Areas of Prisms
Find the lateral area and surface area of the right rectangular prism. Round to the nearest tenth, if necessary.

Example 1B: Finding Lateral Areas and Surface Areas of Prisms
Find the lateral area and surface area of a right regular triangular prism with height 20 cm and base edges of length 10 cm. Round to the nearest tenth, if necessary.

The lateral surface of a cylinder is the curved surface that connects the two bases. The axis of a cylinder is the segment with endpoints at the centers of the bases. The axis of a right cylinder is perpendicular to its bases. The axis of an oblique cylinder is not perpendicular to its bases. The altitude of a right cylinder is the same length as the axis.

Example 2A: Finding Lateral Areas and Surface Areas of Right Cylinders
Find the lateral area and surface area of the right cylinder. Give your answers in terms of . The radius is half the diameter, or 8 in. L = 2rh = 2(8)(10) = 160 in2 S = L + 2r2 = 160 + 2(8)2 = 288 in2

Example 2B: Finding Lateral Areas and Surface Areas of Right Cylinders
Find the lateral area and surface area of a right cylinder with circumference 24 cm and a height equal to half the radius. Give your answers in terms of . Step 1 Use the circumference to find the radius. C = 2r Circumference of a circle 24 = 2r Substitute 24 for C. r = 12 Divide both sides by 2.

Example 2B Continued Find the lateral area and surface area of a right cylinder with circumference 24 cm and a height equal to half the radius. Give your answers in terms of . Step 2 Use the radius to find the lateral area and surface area. The height is half the radius, or 6 cm. L = 2rh = 2(12)(6) = 144 cm2 Lateral area S = L + 2r2 = 144 + 2(12)2 = 432 in2 Surface area

Check It Out! Example 2 Continued
Find the lateral area and surface area of a cylinder with a base area of 49 and a height that is 2 times the radius.

Example 3: Finding Surface Areas of Composite Three-Dimensional Figures
Find the surface area of the composite figure.

Example 3 Continued The surface area of the rectangular prism is . A right triangular prism is added to the rectangular prism. The surface area of the triangular prism is . Two copies of the rectangular prism base are removed. The area of the base is B = 2(4) = 8 cm2.

Example 3 Continued The surface area of the composite figure is the sum of the areas of all surfaces on the exterior of the figure. S = (rectangular prism surface area) + (triangular prism surface area) – 2(rectangular prism base area) S = – 2(8) = 72 cm2

Check It Out! Example 3 Find the surface area of the composite figure. Round to the nearest tenth.

Check It Out! Example 3 Continued
Find the surface area of the composite figure. Round to the nearest tenth. The surface area of the rectangular prism is S =Ph + 2B = 26(5) + 2(36) = 202 cm2. The surface area of the cylinder is S =Ph + 2B = 2(2)(3) + 2(2)2 = 20 ≈ 62.8 cm2. The surface area of the composite figure is the sum of the areas of all surfaces on the exterior of the figure.

Check It Out! Example 3 Continued
Find the surface area of the composite figure. Round to the nearest tenth. S = (rectangular surface area) + (cylinder surface area) – 2(cylinder base area) S = — 2()(22) = cm2

Always round at the last step of the problem
Always round at the last step of the problem. Use the value of  given by the  key on your calculator. Remember!

Example 4: Exploring Effects of Changing Dimensions
The edge length of the cube is tripled. Describe the effect on the surface area.

Example 4 Continued original dimensions: edge length tripled: S = 6ℓ2
24 cm original dimensions: edge length tripled: S = 6ℓ2 S = 6ℓ2 = 6(8)2 = 384 cm2 = 6(24)2 = 3456 cm2 Notice than 3456 = 9(384). If the length, width, and height are tripled, the surface area is multiplied by 32, or 9.

Check It Out! Example 4 The height and diameter of the cylinder are multiplied by . Describe the effect on the surface area.

Check It Out! Example 4 Continued
11 cm 7 cm original dimensions: height and diameter halved: S = 2(112) + 2(11)(14) S = 2(5.52) + 2(5.5)(7) = 550 cm2 = 137.5 cm2 Notice than 550 = 4(137.5). If the dimensions are halved, the surface area is multiplied by

Lesson Quiz: Part I Find the lateral area and the surface area of each figure. Round to the nearest tenth, if necessary. 1. a cube with edge length 10 cm 2. a regular hexagonal prism with height 15 in. and base edge length 8 in. 3. a right cylinder with base area 144 cm2 and a height that is the radius

Lesson Quiz: Part II 4. A cube has edge length 12 cm. If the edge length of the cube is doubled, what happens to the surface area? 5. Find the surface area of the composite figure.