12'3 Surface Area of Pyramids and Cones

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12.3 Surface Area of Pyramids and Cones

• Geometry
• Mrs. Spitz
• Spring 2006

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Objectives/Assignment

• Find the surface area of a pyramid.
• Find the surface area of a cone.
• Assignment WS 12.3A

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Finding the surface area of a pyramid

• A pyramid is a polyhedron in which the base is a
  polygon and the lateral faces are triangles with
  a common vertex. The intersection of two lateral
  faces is a lateral edge. The intersection of the
  base and a lateral face is a base edge. The
  altitude or height of a pyramid is the
  perpendicular distance between the base and the
  vertex.

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More on pyramids

• A regular pyramid has a regular polygon for a
  base and its height meets the base at its center.
  The slant height of a regular pyramid is the
  altitude of any lateral face. A nonregular
  pyramid does not have a slant height.

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Pyramid Arena
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Ex. 1 Finding the Area of a Lateral Face

• Architecture. The lateral faces of the Pyramid
  Arena in Memphis, Tennessee, are covered with
  steal panels. Use the diagram of the arena to
  find the area of each lateral face of this
  regular pyramid.

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Hexagonal Pyramids

• A regular hexagonal pyramid and its net are shown
  at the right. Let b represent the length of a
  base edge, and let l represent the slant height
  of the pyramid. The area of each lateral face is
  1/2bl and the perimeter of the base if P 6b.
  So the surface area is as follows

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Hexagonal pyramid
S (Area of base) 6(Area of lateral face)
S B 6( ½ bl)
Substitute
Rewrite 6( ½ bl) as ½ (6b)l.
S B (6b)l
Substitute P for 6b
S B Pl
Surface Area of a Regular Pyramid
The surface area S of a regular pyramid is S B
½ Pl, where B is the area of the base, P is the
perimeter of the base, and l is the slant height.
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Ex. 2 Finding the surface area of a pyramid

• To find the surface area of the regular pyramid
  shown, start by finding the area of the base.
• Use the formula for the area of a regular
  polygon,
• ½ (apothem)(perimeter). A diagram of the base is
  shown to the right.

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Ex. 2 Finding the surface area of a pyramid

• After substituting, the area of the base is ½ (3
  )(6 6), or
• square meters.

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Surface area

• Now you can find the surface area by using 54
  for the area of the base, B.

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Finding the Surface Area of a Cone

• A circular cone, or cone, has a circular base and
  a vertex that is NOT in the same plane as the
  base. The altitude, or height, is the
  perpendicular distance between the vertex and the
  base. In a right cone, the height meets the base
  at its center and the slant height is the
  distance between the vertex and a point on the
  base edge.

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Finding the Surface Area of a Cone

• The lateral surface of a cone consists of all
  segments that connect the vertex with points on
  the base edge. When you cut along the slant
  height and like the cone flat, you get the net
  shown at the right. In the net, the circular
  base has an area of ?r2 and the lateral surface
  area is the sector of a circle.

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More on cones . . .

• You can find the area of this sector by using a
  proportion, as shown below.

Area of sector
Arc length

Set up proportion
Area of circle
Circumference
2?r
Area of sector
Substitute

?l2
2?l
2?r
Multiply each side by ?l2
Area of sector ?l2
2?l
Area of sector ?rl
Simplify
?The surface area of a cone is the sum of the
base area and the lateral area, ?rl.
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Theorem

• Surface Area of a Right Cone
• The surface area S of a right cone is S ?r2
  ?rl, where r is the radius of the base and l is
  the slant height

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Ex. 3 Finding the surface area of a cone

• To find the surface area of the right cone shown,
  use the formula for the surface area.

S ?r2 ?rl
Write formula
S ?42 ?(4)(6)
Substitute
S 16? 24?
Simplify
S 40?
Simplify
?The surface area is 40? square inches or about
125.7 square inches.
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Upcoming

• There is a quiz after 12.3. There are no other
  quizzes or tests for Chapter 12
• Review for final exam.
• Final Exams Scheduled for Wednesday, May 24.
  You must take and pass the final exam to pass the
  course!
• Book return You will turn in books/CDs this
  date. No book returned F for semester! Book
  is 75 to replace.
• Absences More than 10 in a semester from
  January 9 to May 26, and I will fail you.
  Tardies count!!!