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NYS COMMON CORE MATHEMATICS CURRICULUM 8โ€ข7 Lesson 19 Lesson 19: Cones and Spheres 261 This work is derived from Eureka Math โ„ข and licensed by Great Minds. ยฉ2015 Great Minds. eureka-math.org This file derived from G8-M7-TE-1.3.0-10.2015 This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. Lesson 19: Cones and Spheres Student Outcomes Students use the Pythagorean theorem to determine an unknown dimension of a cone or a sphere. Students know that a pyramid is a special type of cone with triangular faces and a polygonal base. Students know how to use the lateral length of a cone and the length of a chord of a sphere to solve problems related to volume. Classwork Exercises 1โ€“2 (5 minutes) Students complete Exercises 1โ€“2 individually. The purpose of these exercises is for students to perform computations that may help them relate the volume formula for a pyramid to the volume formula for a right circular cone, introduced in Module 5. Their response to part (b) of Exercise 2 is the starting point of the discussion that follows. Exercises 1โ€“2 Note: Figures not drawn to scale. 1. Determine the volume for each figure below. a. Write an expression that shows volume in terms of the area of the base, , and the height of the figure. Explain the meaning of the expression, and then use it to determine the volume of the figure. = The expression = means that the volume of the cylinder is found by multiplying the area of the base by the height. The base is a circle whose area can be found by squaring the radius, ., and then multiplying by . The volume is found by multiplying that area by the height of . = () () = The volume of the cylinder is . Scaffolding: Printable nets, located at the end of the lesson, can be used to create 3-D models for Exercises 1โ€“2.
Transcript
Page 1: Lesson 19: Cones and Spheres - Amazon Web Services

NYS COMMON CORE MATHEMATICS CURRICULUM 8โ€ข7 Lesson 19

Lesson 19: Cones and Spheres

261

This work is derived from Eureka Math โ„ข and licensed by Great Minds. ยฉ2015 Great Minds. eureka-math.org This file derived from G8-M7-TE-1.3.0-10.2015

This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.

Lesson 19: Cones and Spheres

Student Outcomes

Students use the Pythagorean theorem to determine an unknown dimension of a cone or a sphere.

Students know that a pyramid is a special type of cone with triangular faces and a polygonal base.

Students know how to use the lateral length of a cone and the length of a chord of a sphere to solve problems

related to volume.

Classwork

Exercises 1โ€“2 (5 minutes)

Students complete Exercises 1โ€“2 individually. The purpose of these exercises is for students to perform computations

that may help them relate the volume formula for a pyramid to the volume formula for a right circular cone, introduced

in Module 5. Their response to part (b) of Exercise 2 is the starting point of the discussion that follows.

Exercises 1โ€“2

Note: Figures not drawn to scale.

1. Determine the volume for each figure below.

a. Write an expression that shows volume in terms of the area of the base, ๐‘ฉ, and the

height of the figure. Explain the meaning of the expression, and then use it to

determine the volume of the figure.

๐‘ฝ = ๐‘ฉ๐’‰

The expression ๐‘ฝ = ๐‘ฉ๐’‰ means that the volume of the cylinder is found by multiplying the area of the base by

the height. The base is a circle whose area can be found by squaring the radius, ๐Ÿ” ๐ข๐ง., and then multiplying by

๐…. The volume is found by multiplying that area by the height of ๐Ÿ๐ŸŽ.

๐‘ฝ = ๐…(๐Ÿ”)๐Ÿ(๐Ÿ๐ŸŽ)

= ๐Ÿ‘๐Ÿ”๐ŸŽ๐…

The volume of the cylinder is ๐Ÿ‘๐Ÿ”๐ŸŽ๐… ๐ข๐ง๐Ÿ‘.

Scaffolding:

Printable nets, located at the end of the lesson, can be used to create 3-D models for Exercises 1โ€“2.

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b. Write an expression that shows volume in terms of the area of the base, ๐‘ฉ, and the height of the figure.

Explain the meaning of the expression, and then use it to determine the volume of the figure.

๐‘ฝ =๐Ÿ

๐Ÿ‘๐‘ฉ๐’‰

The expression ๐‘ฝ =๐Ÿ๐Ÿ‘

๐‘ฉ๐’‰ means that the volume of the cone is found by

multiplying the area of the base by the height and then taking one-third

of that product. The base is a circle whose area can be found by squaring

the radius, ๐Ÿ” ๐ข๐ง., and then multiplying by ๐…. The volume is found by

multiplying that area by the height of ๐Ÿ๐ŸŽ ๐ข๐ง. and then taking one-third of

that product.

๐‘ฝ =๐Ÿ

๐Ÿ‘๐…(๐Ÿ”)๐Ÿ(๐Ÿ๐ŸŽ)

=๐Ÿ‘๐Ÿ”๐ŸŽ

๐Ÿ‘๐…

= ๐Ÿ๐Ÿ๐ŸŽ ๐…

The volume of the cone is ๐Ÿ๐Ÿ๐ŸŽ๐… ๐ข๐ง๐Ÿ‘.

2.

a. Write an expression that shows volume in terms of the area of the base, ๐‘ฉ, and the height of the figure.

Explain the meaning of the expression, and then use it to determine the volume of the figure.

๐‘ฝ = ๐‘ฉ๐’‰

The expression ๐‘ฝ = ๐‘ฉ๐’‰ means that the volume of the prism is

found by multiplying the area of the base by the height. The base

is a square whose area can be found by multiplying ๐Ÿ๐Ÿ ร— ๐Ÿ๐Ÿ. The

volume is found by multiplying that area, ๐Ÿ๐Ÿ’๐Ÿ’, by the height of ๐Ÿ๐ŸŽ.

๐‘ฝ = ๐Ÿ๐Ÿ(๐Ÿ๐Ÿ)(๐Ÿ๐ŸŽ)

= ๐Ÿ, ๐Ÿ’๐Ÿ’๐ŸŽ

The volume of the prism is ๐Ÿ, ๐Ÿ’๐Ÿ’๐ŸŽ ๐ข๐ง๐Ÿ‘.

b. The volume of the square pyramid shown below is ๐Ÿ’๐Ÿ–๐ŸŽ ๐ข๐ง๐Ÿ‘. What might be a reasonable guess for the

formula for the volume of a pyramid? What makes you suggest your particular guess?

Since ๐Ÿ’๐Ÿ–๐ŸŽ =๐Ÿ๐Ÿ’๐Ÿ’๐ŸŽ

๐Ÿ‘, the formula to find the volume of a pyramid is

likely ๐Ÿ

๐Ÿ‘๐‘ฉ๐’‰, where ๐‘ฉ is the area of the base. This is similar to the

volume of a cone compared to the volume of a cylinder with the

same base and height. The volume of a square pyramid is ๐Ÿ

๐Ÿ‘ of the

volume of the rectangular prism with the same base and height.

MP.7 &

MP.8

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This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.

Discussion (5 minutes)

What do you think the formula to find the volume of a pyramid is? Explain.

Ask students to share their response to part (b) of Exercise 2. If students do not see the connection between cones and

cylinders to pyramids and prisms, then use the discussion points below.

A pyramid is similar to a cone, but a pyramid has a polygonal base and faces that are shaped like triangles.

For now we focus on right square pyramids only, that is, pyramids that have a base that is a square.

The relationship between a cone and cylinder is similar for pyramids and prisms. How are the volumes of

cones and cylinders related?

A cone is one-third the volume of a cylinder with the same base and height.

In general, we say that the volume of a cylinder is ๐‘‰ = ๐ตโ„Ž, where ๐ต is the area of the base. Then the volume

of a cone is ๐‘‰ =13

๐ตโ„Ž, again where ๐ต is the area of the base.

How do you think the volumes of rectangular pyramids and rectangular prisms are related?

The volume of a rectangular pyramid is one-third the volume of a rectangular prism with the same base

and height.

In general, the volume of a rectangular prism is ๐‘‰ = ๐ตโ„Ž, where ๐ต is the area of the base. Then the volume of a

pyramid is ๐‘‰ =13

๐ตโ„Ž, again where ๐ต is the area of the base.

Example 1

Example 1

State as many facts as you can about a cone.

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This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.

Provide students with a minute or two to discuss as many facts as they can about a right circular cone, and then have

them share their facts with the class. As they identify parts of the cone and facts about the cone, label the drawing

above. Students should be able to state or identify the following: radius, diameter, height, base, area of a circle is

๐ด = ๐œ‹๐‘Ÿ2, circumference of a circle is ๐ถ = 2๐œ‹๐‘Ÿ = 2๐‘‘, and the volume of a cone is ๐‘‰ =13

๐ตโ„Ž, where ๐ต is the area of the

base.

What part of the cone have we not identified?

The slanted part of the cone

The slanted part of the cone is known as the lateral length, which is also referred to as the slant height. We

denote the lateral length of a cone by ๐‘ .

Label the lateral length of the cone with ๐‘  on the drawing on the previous page.

Now that we know about the lateral length of a cone, we can begin using it in our work.

Exercises 3โ€“6 (9 minutes)

Students work in pairs to complete Exercises 3โ€“6. Students may need assistance determining the dimensions of the

various parts of a cone. Let students reason through it first, offering guidance if necessary. Consider allowing students

to use a calculator to approximate their answers or allow students to leave their answers as square roots, so as not to

distract from the goal of the lesson. As needed, continue to remind students that we need only consider the positive

square root of a number when our context involves length.

Exercises 3โ€“10

3. What is the lateral length (slant height) of the cone shown below?

Let ๐’„ be the lateral length.

๐Ÿ‘๐Ÿ + ๐Ÿ’๐Ÿ = ๐’„๐Ÿ

๐Ÿ— + ๐Ÿ๐Ÿ” = ๐’„๐Ÿ

๐Ÿ๐Ÿ“ = ๐’„๐Ÿ

โˆš๐Ÿ๐Ÿ“ = โˆš๐’„๐Ÿ

๐Ÿ“ = ๐’„

The lateral length of the cone is ๐Ÿ“ ๐ฎ๐ง๐ข๐ญ๐ฌ.

4. Determine the exact volume of the cone shown below.

Let ๐ซ be the radius of the base.

๐Ÿ”๐Ÿ + ๐’“๐Ÿ = ๐Ÿ—๐Ÿ

๐Ÿ‘๐Ÿ” + ๐’“๐Ÿ = ๐Ÿ–๐Ÿ

๐’“๐Ÿ = ๐Ÿ’๐Ÿ“

The area of the base is ๐Ÿ’๐Ÿ“๐… ๐ฎ๐ง๐ข๐ญ๐ฌ๐Ÿ.

๐‘ฝ =๐Ÿ

๐Ÿ‘๐‘ฉ๐’‰

๐‘ฝ =๐Ÿ

๐Ÿ‘(๐Ÿ’๐Ÿ“)๐…(๐Ÿ”)

๐‘ฝ = ๐Ÿ—๐ŸŽ๐…

The volume of the cone is ๐Ÿ—๐ŸŽ๐… ๐ฎ๐ง๐ข๐ญ๐ฌ๐Ÿ‘.

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This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.

5. What is the lateral length (slant height) of the pyramid shown below? Give an exact square root answer and an

approximate answer rounded to the tenths place.

Let ๐’„ ๐ข๐ง. represent the lateral length of the pyramid.

๐Ÿ’๐Ÿ + ๐Ÿ–๐Ÿ = ๐’„๐Ÿ ๐Ÿ๐Ÿ” + ๐Ÿ”๐Ÿ’ = ๐’„๐Ÿ

๐Ÿ–๐ŸŽ = ๐’„๐Ÿ

โˆš๐Ÿ–๐ŸŽ = โˆš๐’„๐Ÿ

โˆš๐Ÿ–๐ŸŽ = ๐’„

The number โˆš๐Ÿ–๐ŸŽ is between ๐Ÿ– and ๐Ÿ—. In the sequence of

tenths, it is between ๐Ÿ–. ๐Ÿ— and ๐Ÿ—. ๐ŸŽ. Since ๐Ÿ–๐ŸŽ is closer to ๐Ÿ–. ๐Ÿ—๐Ÿ

than ๐Ÿ—๐Ÿ, the approximate lateral length is ๐Ÿ–. ๐Ÿ— inches.

6. Determine the volume of the square pyramid shown below. Give an exact answer using a square root.

Let ๐’‰ be the height of the pyramid.

๐Ÿ๐Ÿ + ๐’‰๐Ÿ = ๐Ÿ๐Ÿ

๐Ÿ + ๐’‰๐Ÿ = ๐Ÿ’

๐’‰๐Ÿ = ๐Ÿ‘

โˆš๐’‰๐Ÿ = โˆš๐Ÿ‘

๐’‰ = โˆš๐Ÿ‘

The area of the base is ๐Ÿ’ ๐ฎ๐ง๐ข๐ญ๐ฌ๐Ÿ.

๐‘ฝ =๐Ÿ

๐Ÿ‘(๐Ÿ’)(โˆš๐Ÿ‘)

=๐Ÿ’โˆš๐Ÿ‘

๐Ÿ‘

The volume of the pyramid is ๐Ÿ’โˆš๐Ÿ‘

๐Ÿ‘ ๐ฎ๐ง๐ข๐ญ๐ฌ๐Ÿ‘.

Discussion (7 minutes)

Let ๐‘‚ be the center of a circle, and let ๐‘ƒ and ๐‘„ be two points on the circle as shown. Then ๐‘ƒ๐‘„ฬ…ฬ… ฬ…ฬ… is called a chord

of the circle.

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This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.

What do you notice about the lengths |๐‘‚๐‘ƒ| and |๐‘‚๐‘„|?

Both lengths are equal to the radius, ๐‘Ÿ, of the circle, which means they are equal in length to each

other.

Will lengths |๐‘‚๐‘ƒ| and |๐‘‚๐‘„| always be equal to ๐‘Ÿ, no matter where the chord is drawn?

Provide students time to place points ๐‘ƒ and ๐‘„ around the circle to get an idea that no matter where the endpoints of the

chord are placed, the length from the center of the circle to each of those points is always equal to ๐‘Ÿ. The reason is

based on the definition of a chord. Points ๐‘ƒ and ๐‘„ must lie on the circle in order for ๐‘ƒ๐‘„ฬ…ฬ… ฬ…ฬ… to be identified as a chord.

When the angle โˆ ๐‘ƒ๐‘‚๐‘„ is a right angle, we can use the Pythagorean theorem to determine the length of the

chord given the length of the radius; or, if we know the length of the chord, we can determine the length of

the radius.

Similarly, when points ๐‘ƒ and ๐‘„ are on the surface of a sphere, the segment that connects them is called a

chord.

Just like with circles, if the angle formed by ๐‘ƒ๐‘‚๐‘„ is a right angle, then we can use the Pythagorean theorem to

find the length of the chord if we are given the length of the radius; or, given the length of the chord, we can

determine the radius of the sphere.

Exercises 7โ€“10 (9 minutes)

Students work in pairs to complete Exercises 7โ€“10. Consider allowing students to use their calculators or to leave their

answers as square roots (simplified square roots if that lesson was used with students), but not approximated, so as not

to distract from the goal of the lesson.

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7. What is the length of the chord of the sphere shown below? Give an exact answer using a square root.

Let ๐’„ ๐œ๐ฆ represent the length of the chord.

๐Ÿ๐Ÿ๐Ÿ + ๐Ÿ๐Ÿ๐Ÿ = ๐’„๐Ÿ

๐Ÿ๐Ÿ๐Ÿ + ๐Ÿ๐Ÿ๐Ÿ = ๐’„๐Ÿ

๐Ÿ๐Ÿ’๐Ÿ = ๐’„๐Ÿ

โˆš๐Ÿ๐Ÿ’๐Ÿ = โˆš๐’„๐Ÿ

โˆš๐Ÿ๐Ÿ๐Ÿ ร— ๐Ÿ = ๐’„

๐Ÿ๐Ÿโˆš๐Ÿ = ๐’„

The length of the chord is โˆš๐Ÿ๐Ÿ’๐Ÿ ๐œ๐ฆ, or ๐Ÿ๐Ÿโˆš๐Ÿ ๐œ๐ฆ.

8. What is the length of the chord of the sphere shown below? Give an exact answer using a square root.

Let ๐’„ ๐ข๐ง. represent the length of the chord.

๐Ÿ’๐Ÿ + ๐Ÿ’๐Ÿ = ๐’„๐Ÿ

๐Ÿ๐Ÿ” + ๐Ÿ๐Ÿ” = ๐’„๐Ÿ

๐Ÿ‘๐Ÿ = ๐’„๐Ÿ

โˆš๐Ÿ‘๐Ÿ = โˆš๐’„๐Ÿ

โˆš๐Ÿ’๐Ÿ ร— ๐Ÿ = ๐’„

๐Ÿ’โˆš๐Ÿ = ๐’„

The length of the chord is โˆš๐Ÿ‘๐Ÿ ๐ข๐ง., or ๐Ÿ’โˆš๐Ÿ ๐ข๐ง.

9. What is the volume of the sphere shown below? Give an exact answer using a square root.

Let ๐’“ ๐œ๐ฆ represent the radius of the sphere.

The volume of the sphere is

๐Ÿ’๐ŸŽ๐ŸŽ๐ŸŽโˆš๐Ÿ–

๐Ÿ‘ ๐… ๐œ๐ฆ๐Ÿ‘, or

๐Ÿ–๐ŸŽ๐ŸŽ๐ŸŽโˆš๐Ÿ

๐Ÿ‘๐… ๐œ๐ฆ๐Ÿ‘.

๐’“๐Ÿ + ๐’“๐Ÿ = ๐Ÿ๐ŸŽ๐Ÿ

๐Ÿ๐’“๐Ÿ = ๐Ÿ’๐ŸŽ๐ŸŽ

๐’“๐Ÿ = ๐Ÿ๐ŸŽ๐ŸŽ

โˆš๐’“๐Ÿ = โˆš๐Ÿ๐ŸŽ๐ŸŽ

๐’“ = โˆš๐Ÿ๐ŸŽ๐ŸŽ

๐’“ = โˆš๐Ÿ๐ŸŽ๐Ÿ ร— ๐Ÿ

๐’“ = ๐Ÿ๐ŸŽโˆš๐Ÿ

๐‘ฝ =๐Ÿ’

๐Ÿ‘๐…๐’“๐Ÿ‘

=๐Ÿ’

๐Ÿ‘๐…(๐Ÿ๐ŸŽโˆš๐Ÿ)

๐Ÿ‘

=๐Ÿ’

๐Ÿ‘๐…(๐Ÿ๐ŸŽ๐Ÿ‘)(โˆš๐Ÿ)

๐Ÿ‘

=๐Ÿ’

๐Ÿ‘๐…(๐Ÿ๐ŸŽ๐ŸŽ๐ŸŽ)(โˆš๐Ÿ–)

=๐Ÿ’

๐Ÿ‘๐…(๐Ÿ๐ŸŽ๐ŸŽ๐ŸŽ) (โˆš๐Ÿ๐Ÿ ร— ๐Ÿ)

=๐Ÿ’

๐Ÿ‘๐…(๐Ÿ๐ŸŽ๐ŸŽ๐ŸŽ)(๐Ÿ)(โˆš๐Ÿ)

=๐Ÿ–๐ŸŽ๐ŸŽ๐ŸŽโˆš๐Ÿ

๐Ÿ‘๐…

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10. What is the volume of the sphere shown below? Give an exact answer using a square root.

Let ๐’“ ๐ฆ๐ฆ represent the radius of the sphere.

๐’“๐Ÿ + ๐’“๐Ÿ = ๐Ÿ๐Ÿ๐Ÿ

๐Ÿ๐’“๐Ÿ = ๐Ÿ๐Ÿ’๐Ÿ’

๐’“๐Ÿ = ๐Ÿ•๐Ÿ

โˆš๐’“๐Ÿ = โˆš๐Ÿ•๐Ÿ

๐’“ = โˆš๐Ÿ”๐Ÿ ร— ๐Ÿ

๐’“ = ๐Ÿ”โˆš๐Ÿ

๐‘ฝ =๐Ÿ’

๐Ÿ‘๐…๐’“๐Ÿ‘

=๐Ÿ’

๐Ÿ‘๐…(๐Ÿ”โˆš๐Ÿ)

๐Ÿ‘

=๐Ÿ’

๐Ÿ‘๐…(๐Ÿ”๐Ÿ‘)(โˆš๐Ÿ)

๐Ÿ‘

=๐Ÿ’

๐Ÿ‘๐…(๐Ÿ๐Ÿ๐Ÿ”)(โˆš๐Ÿ–)

=๐Ÿ’

๐Ÿ‘๐…(๐Ÿ๐Ÿ๐Ÿ”) (โˆš๐Ÿ๐Ÿ ร— ๐Ÿ)

=๐Ÿ’

๐Ÿ‘๐…(๐Ÿ๐Ÿ๐Ÿ”)(๐Ÿ)(โˆš๐Ÿ)

=๐Ÿ๐Ÿ•๐Ÿ๐Ÿ–โˆš๐Ÿ

๐Ÿ‘๐…

= ๐Ÿ“๐Ÿ•๐Ÿ”โˆš๐Ÿ๐…

The volume of the sphere is ๐Ÿ“๐Ÿ•๐Ÿ”โˆš๐Ÿ๐… ๐ฆ๐ฆ๐Ÿ‘.

Closing (5 minutes)

Summarize, or ask students to summarize, the main points from the lesson.

The volume formulas for rectangular pyramids and rectangular prisms are similar to those of cones and

cylinders.

The formula to determine the volume of a pyramid is 1

3๐ตโ„Ž, where ๐ต is the area of the base. This is similar to

the formula to determine the volume of a cone.

The segment formed by two points on a circle is called a chord.

We know how to apply the Pythagorean theorem to cones and spheres to determine volume.

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Exit Ticket (5 minutes)

Lesson Summary

The volume formula for a right square pyramid is ๐‘ฝ =๐Ÿ๐Ÿ‘

๐‘ฉ๐’‰, where ๐‘ฉ is the area of the square base.

The lateral length of a cone, sometimes referred to as the slant height, is the side ๐’”, shown in the diagram below.

Given the lateral length and the length of the radius, the Pythagorean theorem can be used to determine the height

of the cone.

Let ๐‘ถ be the center of a circle, and let ๐‘ท and ๐‘ธ be two points on the circle. Then ๐‘ท๐‘ธฬ…ฬ… ฬ…ฬ… is called a chord of the circle.

The segments ๐‘ถ๐‘ท and ๐‘ถ๐‘ธ are equal in length because both represent the radius of the circle. If the angle formed

by ๐‘ท๐‘ถ๐‘ธ is a right angle, then the Pythagorean theorem can be used to determine the length of the radius when

given the length of the chord, or the length of the chord can be determined if given the length of the radius.

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Name Date

Lesson 19: Cones and Spheres

Exit Ticket

Which has the larger volume? Give an approximate answer rounded to the tenths place.

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Exit Ticket Sample Solutions

Which has the larger volume? Give an approximate answer rounded to the tenths place.

Let ๐’‰ ๐œ๐ฆ represent the height of the square pyramid.

๐’‰๐Ÿ + ๐Ÿ”๐Ÿ = ๐Ÿ๐ŸŽ๐Ÿ

๐’‰๐Ÿ + ๐Ÿ‘๐Ÿ” = ๐Ÿ๐ŸŽ๐ŸŽ

๐’‰๐Ÿ = ๐Ÿ”๐Ÿ’

๐’‰ = ๐Ÿ–

๐‘ฝ

๐‘ฝ

๐‘ฝ = ๐Ÿ‘๐Ÿ–๐Ÿ’

The volume of the square pyramid is ๐Ÿ‘๐Ÿ–๐Ÿ’ ๐œ๐ฆ๐Ÿ‘.

Let ๐’“ represent the radius of the sphere in centimeters.

๐’“๐Ÿ + ๐’“๐Ÿ = ๐Ÿ”๐Ÿ

๐Ÿ๐’“๐Ÿ = ๐Ÿ‘๐Ÿ”

๐’“๐Ÿ = ๐Ÿ๐Ÿ–

โˆš๐’“๐Ÿ = โˆš๐Ÿ๐Ÿ–

๐’“ = โˆš๐Ÿ‘๐Ÿ ร— ๐Ÿ

๐’“ = ๐Ÿ‘โˆš๐Ÿ

๐‘ฝ =๐Ÿ’

๐Ÿ‘๐…๐’“๐Ÿ‘

=๐Ÿ’

๐Ÿ‘๐…(๐Ÿ‘โˆš๐Ÿ)

๐Ÿ‘

=๐Ÿ’

๐Ÿ‘๐…(๐Ÿ‘)๐Ÿ‘(โˆš๐Ÿ)

๐Ÿ‘

=๐Ÿ’

๐Ÿ‘๐…(๐Ÿ๐Ÿ•)(โˆš๐Ÿ–)

=๐Ÿ’

๐Ÿ‘๐…(๐Ÿ๐Ÿ•) (โˆš๐Ÿ๐Ÿ ร— ๐Ÿ)

=๐Ÿ’

๐Ÿ‘๐…(๐Ÿ๐Ÿ•)(๐Ÿ)(โˆš๐Ÿ)

= ๐Ÿ•๐Ÿ๐…โˆš๐Ÿ

The volume of the sphere is ๐Ÿ•๐Ÿ๐…โˆš๐Ÿ ๐œ๐ฆ๐Ÿ‘.

The number โˆš๐Ÿ is between ๐Ÿ and ๐Ÿ. In the sequence of tenths, it is between ๐Ÿ. ๐Ÿ’ and ๐Ÿ. ๐Ÿ“. Since ๐Ÿ is closer to ๐Ÿ. ๐Ÿ’๐Ÿ than

๐Ÿ. ๐Ÿ“๐Ÿ, the number is approximately ๐Ÿ. ๐Ÿ’.

We know from previous lessons we can estimate ๐… = ๐Ÿ‘. ๐Ÿ๐Ÿ’.

Then, we can calculate the approximate volume of the sphere:

๐‘ฝ โ‰ˆ (๐Ÿ•๐Ÿ)(๐Ÿ. ๐Ÿ’)(๐Ÿ‘. ๐Ÿ๐Ÿ’)

๐‘ฝ โ‰ˆ ๐Ÿ‘๐Ÿ๐Ÿ”. ๐Ÿ“๐Ÿ๐Ÿ

The approximate volume of the sphere is ๐Ÿ‘๐Ÿ๐Ÿ”. ๐Ÿ“๐Ÿ๐Ÿ ๐œ๐ฆ๐Ÿ‘. Therefore, the volume of the square pyramid is greater.

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Problem Set Sample Solutions

Students use the Pythagorean theorem to solve mathematical problems in three dimensions.

1. What is the lateral length (slant height) of the cone shown below? Give an approximate answer rounded to the

tenths place.

Let ๐’„ ๐ฆ be the lateral length.

๐Ÿ๐ŸŽ๐Ÿ + ๐Ÿ’๐Ÿ = ๐’„๐Ÿ

๐Ÿ๐ŸŽ๐ŸŽ + ๐Ÿ๐Ÿ” = ๐’„๐Ÿ

๐Ÿ๐Ÿ๐Ÿ” = ๐’„๐Ÿ

โˆš๐Ÿ๐Ÿ๐Ÿ” = โˆš๐’„๐Ÿ

โˆš๐Ÿ๐Ÿ๐Ÿ” = ๐’„

The number โˆš๐Ÿ๐Ÿ๐Ÿ” is between ๐Ÿ๐ŸŽ and ๐Ÿ๐Ÿ. In the sequence of tenths, it is between

๐Ÿ๐ŸŽ. ๐Ÿ• and ๐Ÿ๐ŸŽ. ๐Ÿ–. Since ๐Ÿ๐Ÿ๐Ÿ” is closer to ๐Ÿ๐ŸŽ. ๐Ÿ–๐Ÿ than ๐Ÿ๐ŸŽ. ๐Ÿ•๐Ÿ, the approximate value

of the number is ๐Ÿ๐ŸŽ. ๐Ÿ–.

The lateral length of the cone is approximately ๐Ÿ๐ŸŽ. ๐Ÿ– ๐ฆ.

2. What is the volume of the cone shown below? Give an exact answer.

Let ๐’‰ represent the height of a cone.

๐Ÿ“๐Ÿ + ๐’‰๐Ÿ = ๐Ÿ๐Ÿ‘๐Ÿ

๐Ÿ๐Ÿ“ + ๐’‰๐Ÿ = ๐Ÿ๐Ÿ”๐Ÿ—

๐’‰๐Ÿ = ๐Ÿ๐Ÿ’๐Ÿ’

โˆš๐’‰๐Ÿ = โˆš๐Ÿ๐Ÿ’๐Ÿ’

๐’‰ = ๐Ÿ๐Ÿ

The height of the cone is ๐Ÿ๐Ÿ ๐ฎ๐ง๐ข๐ญ๐ฌ.

๐‘ฝ =๐Ÿ

๐Ÿ‘๐…(๐Ÿ๐Ÿ“)(๐Ÿ๐Ÿ)

= ๐Ÿ๐ŸŽ๐ŸŽ๐…

The volume of the cone is ๐ŸŽ๐ŸŽ๐… ๐ฎ๐ง๐ข๐ญ๐ฌ๐Ÿ‘.

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3. Determine the volume and surface area of the square pyramid shown below. Give exact answers.

๐‘ฝ =๐Ÿ

๐Ÿ‘(๐Ÿ”๐Ÿ’)(๐Ÿ•)

=๐Ÿ’๐Ÿ’๐Ÿ–

๐Ÿ‘

The volume of the pyramid is ๐Ÿ’๐Ÿ’๐Ÿ–

๐Ÿ‘ ๐ฎ๐ง๐ข๐ญ๐ฌ๐Ÿ‘.

Let ๐’„ represent the lateral length.

๐Ÿ•๐Ÿ + ๐Ÿ’๐Ÿ = ๐’„๐Ÿ

๐Ÿ’๐Ÿ— + ๐Ÿ๐Ÿ” = ๐’„๐Ÿ

๐Ÿ”๐Ÿ“ = ๐’„๐Ÿ

โˆš๐Ÿ”๐Ÿ“ = โˆš๐’„๐Ÿ

โˆš๐Ÿ”๐Ÿ“ = ๐’„

The area of each face of the pyramid is ๐Ÿ’โˆš๐Ÿ”๐Ÿ“ ๐ฎ๐ง๐ข๐ญ๐ฌ๐Ÿ (since ๐Ÿ๐Ÿ

ร— ๐Ÿ– ร— โˆš๐Ÿ”๐Ÿ“ = ๐Ÿ’โˆš๐Ÿ”๐Ÿ“), so the area of all four faces is

๐Ÿ”โˆš๐Ÿ”๐Ÿ“ ๐ฎ๐ง๐ข๐ญ๐ฌ๐Ÿ. Since the base area is ๐Ÿ’ ๐ฎ๐ง๐ข๐ญ๐ฌ๐Ÿ, the total surface area of the pyramid is (๐Ÿ”๐Ÿ’ + ๐Ÿ๐Ÿ”โˆš๐Ÿ”๐Ÿ“) ๐ฎ๐ง๐ข๐ญ๐ฌ๐Ÿ.

4. Alejandra computed the volume of the cone shown below as ๐Ÿ”๐Ÿ’๐… ๐œ๐ฆ๐Ÿ‘. Her work is shown below. Is she correct?

If not, explain what she did wrong, and calculate the correct volume of the cone. Give an exact answer.

The volume of the cone is ๐Ÿ”๐Ÿ’๐… ๐œ๐ฆ๐Ÿ‘.

Alejandraโ€™s work is incorrect. She used the lateral length instead of the height of the cone to compute volume.

Let ๐’‰ ๐œ๐ฆ represent the height.

๐Ÿ’๐Ÿ + ๐’‰๐Ÿ = ๐Ÿ๐Ÿ๐Ÿ

๐Ÿ๐Ÿ” + ๐’‰๐Ÿ = ๐Ÿ๐Ÿ’๐Ÿ’

๐’‰๐Ÿ = ๐Ÿ๐Ÿ๐Ÿ–

โˆš๐’‰๐Ÿ = โˆš๐Ÿ๐Ÿ๐Ÿ–

๐’‰ = โˆš๐Ÿ๐Ÿ๐Ÿ–

๐’‰ = โˆš๐Ÿ–๐Ÿ ร— ๐Ÿ

๐’‰ = ๐Ÿ–โˆš๐Ÿ

๐‘ฝ =๐Ÿ

๐Ÿ‘๐…(๐Ÿ’)๐Ÿ(๐Ÿ–โˆš๐Ÿ)

๐‘ฝ =๐Ÿ

๐Ÿ‘๐…(๐Ÿ๐Ÿ๐Ÿ–โˆš๐Ÿ)

๐‘ฝ =๐Ÿ๐Ÿ๐Ÿ–โˆš๐Ÿ

๐Ÿ‘๐…

The volume of the cone is ๐Ÿ๐Ÿ๐Ÿ–โˆš๐Ÿ

๐Ÿ‘๐… ๐œ๐ฆ๐Ÿ‘.

๐‘ฝ =๐Ÿ

๐Ÿ‘๐…(๐Ÿ’๐Ÿ)(๐Ÿ๐Ÿ)

=(๐Ÿ๐Ÿ”)(๐Ÿ๐Ÿ)๐…

๐Ÿ‘

= ๐Ÿ”๐Ÿ’๐…

= ๐Ÿ”๐Ÿ’

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5. What is the length of the chord of the sphere shown below? Give an exact answer using a square root.

Let ๐’„ ๐ฆ represent the length of the chord.

๐Ÿ—๐Ÿ + ๐Ÿ—๐Ÿ = ๐’„๐Ÿ

๐Ÿ–๐Ÿ + ๐Ÿ–๐Ÿ = ๐’„๐Ÿ

๐Ÿ๐Ÿ”๐Ÿ = ๐’„๐Ÿ

โˆš๐Ÿ๐Ÿ”๐Ÿ = โˆš๐’„๐Ÿ

โˆš๐Ÿ๐Ÿ”๐Ÿ = ๐’„

โˆš๐Ÿ—๐Ÿ ร— ๐Ÿ = ๐’„

๐Ÿ—โˆš๐Ÿ = ๐’„

The length of the chord is โˆš๐Ÿ๐Ÿ”๐Ÿ ๐ฆ, or ๐Ÿ—โˆš๐Ÿ ๐ฆ.

6. What is the volume of the sphere shown below? Give an exact answer using a square root.

Let ๐’“ ๐ข๐ง. represent the radius.

๐’“๐Ÿ + ๐’“๐Ÿ = ๐Ÿ๐Ÿ’๐Ÿ

๐Ÿ๐’“๐Ÿ = ๐Ÿ๐Ÿ—๐Ÿ”

๐’“๐Ÿ = ๐Ÿ—๐Ÿ–

โˆš๐’“๐Ÿ = โˆš๐Ÿ—๐Ÿ–

๐’“ = โˆš๐Ÿ•๐Ÿ ร— ๐Ÿ

๐’“ = ๐Ÿ•โˆš๐Ÿ

๐‘ฝ =๐Ÿ’

๐Ÿ‘๐…๐’“๐Ÿ‘

=๐Ÿ’

๐Ÿ‘๐…(๐Ÿ•โˆš๐Ÿ)

๐Ÿ‘

=๐Ÿ’

๐Ÿ‘๐…(๐Ÿ‘๐Ÿ’๐Ÿ‘)(โˆš๐Ÿ–)

=๐Ÿ’

๐Ÿ‘๐…(๐Ÿ‘๐Ÿ’๐Ÿ‘)(๐Ÿโˆš๐Ÿ)

=๐Ÿ๐Ÿ•๐Ÿ’๐Ÿ’โˆš๐Ÿ

๐Ÿ‘๐…

The volume of the sphere is ๐Ÿ’๐Ÿ‘

(โˆš๐Ÿ—๐Ÿ–)๐Ÿ‘

๐… ๐ข๐ง๐Ÿ‘, or ๐Ÿ๐Ÿ•๐Ÿ’๐Ÿ’โˆš๐Ÿ

๐Ÿ‘๐… ๐ข๐ง๐Ÿ‘.

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