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**12.4 Volume of Prisms & Cylinders**

Geometry Mrs. Spitz Spring 2006

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**Objectives/Assignment**

Use volume postulates Find the volume of prism and cylinders in real life such as concrete blocks or volumes of fish tanks. WS 12.4 A

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Exploring Volume The volume of a solid is the number of cubic units contained in its interior. Volume is measured in cubic units, such as cubic meters (m3).

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**Volume Postulates Volume of a cube Volume Congruence Postulate**

V = s3 Volume Congruence Postulate If two polyhedra are congruent, then they have the same volume. Volume Addition Postulate The volume of a solid is the sum of the volumes of all its nonoverlapping parts.

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**Ex. 1: Finding the Volume of a rectangular prism**

The box shown is 5 units long, 3 units wide, and 4 units high. How many unit cubes will fit in the box? What is the volume of the box?

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**Ex. 1: Finding the Volume of a rectangular prism**

The base of the box is 5 units by 3 units. This means 5 • 3, or 15 unit cubes, will cover the base. Three more layers of 15 cubes each can be placed on top of the lower layer to fill the box. Because the box contains 4 layers with 15 cubes in each layer, the box contains a total of 4 •15 cubes, or 60 unit cubes.

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Conclusion Because the box is completely filled by the 60 cubes, and each cube has a volume of 1 cubic unit, it follows that the volume of the box is 60 • 1, or 60 cubic units.

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Note: In example 1, the area of the base, 15 square units, multiplied by the height of 4 units, yields the volume of the box, 60 cubic units. So, the volume of the prism can be found by multiplying the area of the base by the height. This method can also be used to find the volume of a cylinder.

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**Finding Volumes of prisms and cylinders.**

Theorem 12.6 is named after Bonaventura Cavalieri ( ). To see how it can be applied, consider the solids on the next slide. All three have cross sections with equal areas, B, and all three have equal heights, h. By Cavalieri’s Principle, it follows that each solid has the same volume.

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**Cavalieri’s Principle**

If two solids have the same height and the same cross-sectional area at every level, then they have the same volume.

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Volume Theorems Volume of a Prism— The volume V of a prism is V=Bh, where B is the area of the base and h is the height. Volume of a Cylinder— The volume V of a cylinder is V=Bh=r2h, where B is the area of a base, h is the height, and r is the radius of the base.

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**Ex. 2: Finding Volumes A = ½ bh Area of a triangle A = ½ (3)(4)**

Find the volume of the right prism. A = ½ bh Area of a triangle A = ½ (3)(4) Substitute values A = 6 cm2 Multiply values -- base V = Bh Volume of a prism formula V = (6)(2) Substitute values V = 12 cm3 Multiply values & solve

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**Ex. 2: Finding Volumes A = r2 Area of a circle A = 82**

Find the volume of the right cylinder. A = r2 Area of a circle A = 82 Substitute values A = 64 in.2 Multiply values -- base V = Bh Volume of a prism formula V = 64(6) Substitute values V = 384in.3 Multiply values & solve V = in.3 Simplify

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Ex. 3: Using Volumes Use the measurements given to solve for x.

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Ex. 3: Using Volumes Use the measurements given to solve for x.

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**Ex. 4: Using Volumes in Real Life**

Construction. Concrete weighs 145 pounds per cubic foot. To find the weight of the concrete block shown, you need to find its volume. The area of the base can be found as follows:

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**Ex. 4: Using Volumes in Real Life**

Using the formula for the volume of a prism, the volume is V = Bh 0.61(0.66) 0.40 ft.3 To find the weight of the block, multiply the pounds per cubic foot, 145 lb/ft.3 by the number of cubic feet, 0.40 ft3. 145 lb • 0.40 ft3 58 lbs. Weight = 1 ft3

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Upcoming There is a quiz after 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/CD’s 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!!!

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