ISEE Upper Level Math : Pyramids

Study concepts, example questions & explanations for ISEE Upper Level Math

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Example Questions

Example Question #1 : Volume Of A Pyramid

A pyramid has height 4 feet. Its base is a square with sidelength 3 feet. Give its volume in cubic inches.

Possible Answers:

\displaystyle 62,208 \textrm{ in}^{3}

\displaystyle 5,184\textrm{ in}^{3}

\displaystyle 124,416\textrm{ in}^{3}

\displaystyle 31,104\textrm{ in}^{3}

\displaystyle 20,736 \textrm{ in}^{3}

Correct answer:

\displaystyle 20,736 \textrm{ in}^{3}

Explanation:

Convert each measurement from inches to feet by multiplying it by 12:

Height: 4 feet = \displaystyle 4 \times 12 = 48 inches

Sidelength of the base: 3 feet = \displaystyle 3 \times 12 = 36 inches

The volume of a pyramid is 

\displaystyle V = \frac{1}{3} Bh

Since the base is a square, we can replace \displaystyle B = s^{2}:

\displaystyle V = \frac{1}{3} s ^{2}h

Substitute \displaystyle s=36, h = 48

\displaystyle V = \frac{1}{3} \cdot 36 ^{2}\cdot 48

\displaystyle V =20,736

The pyramid has volume 20,736 cubic inches.

 

Example Question #1 : How To Find The Volume Of A Pyramid

A \displaystyle 300 foot tall pyramid has a square base measuring \displaystyle 40 feet on each side. What is the volume of the pyramid?

Possible Answers:

\displaystyle 160,000 ft^{2}

\displaystyle 4,000 ft^{3}

\displaystyle 160,000 ft^{3}

\displaystyle 12,000 ft^{3}

\displaystyle 480,000 ft^{3}

Correct answer:

\displaystyle 160,000 ft^{3}

Explanation:

In order to find the area of a triangle, we use the formula \displaystyle \frac{1}{3}Bh.  In this case, since the base is a square, we can replace \displaystyle B with \displaystyle s^{2}, so our formula for volume is \displaystyle \frac{1}{3}s^{2}h.

Since the length of each side of the base is \displaystyle 40 feet, we can substitute it in for \displaystyle s.

\displaystyle \frac{1}{3}\cdot (40)^{2}\cdot h

We also know that the height is \displaystyle 300 feet, so we can substitute that in for \displaystyle h.

\displaystyle \frac{1}{3}\cdot (40)^{2}\cdot 300

This gives us an answer of \displaystyle 160,000 ft^{3}.

It is important to remember that volume is expressed in units cubed.

Example Question #1 : Solid Geometry

The height of a right pyramid is \displaystyle 2 feet. Its base is a square with sidelength \displaystyle 3 feet. Give its volume in cubic inches.

Possible Answers:

\displaystyle 15,552 \textrm{ in}^{3}

\displaystyle 41,472 \textrm{ in}^{3}

\displaystyle 10,368 \textrm{ in}^{3}

\displaystyle 62,208\textrm{ in}^{3}

\displaystyle 31,104 \textrm{ in}^{3}

Correct answer:

\displaystyle 10,368 \textrm{ in}^{3}

Explanation:

Convert each of the measurements from feet to inches by multiplying by \displaystyle 12.

Height: \displaystyle 2 \times 12 = 24 inches

Sidelength of base: \displaystyle 3 \times 12 = 36 inches

The base of the pyramid has area 

\displaystyle B = s^{2} = 36^{2} = 1,296 square inches.

Substitute \displaystyle B = 1,296, h = 24  into the volume formula:

\displaystyle V = \frac {1}{3} Bh

\displaystyle V = \frac {1}{3} \cdot 1,296 \cdot 24 =10,368 cubic inches

Example Question #1 : How To Find The Volume Of A Pyramid

The height of a right pyramid is  inches. Its base is a square with sidelength  inches. Give its volume in cubic feet.

Possible Answers:

\displaystyle 9 \frac {1}{3} \textrm{ ft}^3

\displaystyle 4 \frac {2}{3} \textrm{ ft}^3

\displaystyle 2 \frac {1}{3} \textrm{ ft}^3

\displaystyle 14 \textrm{ ft}^3

\displaystyle 4 \textrm{ ft}^3

Correct answer:

\displaystyle 4 \frac {2}{3} \textrm{ ft}^3

Explanation:

Convert each of the measurements from inches to feet by dividing by .

Height: \displaystyle 42 \div 12 = 3 \frac{1}{2} feet

Sidelength: \displaystyle 24 \div 12 = 2 feet

The base of the pyramid has area 

\displaystyle B = s^{2} = 2^{2} = 4 square feet.

Substitute \displaystyle B = 4, h = 3 \frac{1}{2}  into the volume formula:

\displaystyle V = \frac {1}{3} Bh

\displaystyle V = \frac {1}{3}\cdot 4 \cdot 3 \frac{1}{2}

\displaystyle V = \frac {1}{3} \cdot \frac {4}{1} \cdot \frac{7}{2}

\displaystyle V = \frac {1}{3} \cdot \frac {2}{1} \cdot \frac{7}{1} = \frac {14}{3} = 4 \frac {2}{3} cubic feet

Example Question #2 : Solid Geometry

The height of a right pyramid and the sidelength of its square base are equal. The perimeter of the base is 3 feet. Give its volume in cubic inches.

Possible Answers:

\displaystyle 27 \textrm{ in}^{3}

\displaystyle 243 \textrm{ in}^{3}

\displaystyle 729 \textrm{ in}^{3}

\displaystyle 81 \textrm{ in}^{3}

\displaystyle 576 \textrm{ in}^{3}

Correct answer:

\displaystyle 243 \textrm{ in}^{3}

Explanation:

The perimeter of the square base, \displaystyle 3 feet, is equivalent to \displaystyle 3 \times 12 = 36 inches; divide by \displaystyle 4 to get the sidelength of the base - and the height: \displaystyle 36 \div 4 = 9 inches. 

The area of the base is therefore \displaystyle B = s^{2} = 9^{2} = 81 square inches. 

In the formula for the volume of a pyramid, substitute \displaystyle B = 81,h=9:

\displaystyle V = \frac{1}{3} Bh = \frac{1}{3}\cdot 81 \cdot 9 = 243 cubic inches.

Example Question #3 : Solid Geometry

What is the volume of a pyramid with the following measurements?

\displaystyle length=7; width=6;height=9;slant\ length=11

Possible Answers:

\displaystyle 462

\displaystyle 378

\displaystyle 126

\displaystyle 154

Correct answer:

\displaystyle 126

Explanation:

The volume of a pyramid can be determined using the following equation:

\displaystyle V=\frac{1}{3}lwh=\frac{1}{3}(7)(6)(9)=126

Example Question #2 : How To Find The Volume Of A Pyramid

A right regular pyramid with volume \displaystyle 1,000 has its vertices at the points 

\displaystyle (0,0,0), (n,0, 0), (n,n,0), (0, n, n), (n,n,n)

where \displaystyle n > 0.

Evaluate \displaystyle n

Possible Answers:

\displaystyle n = 10 \sqrt[3]{3 }

\displaystyle n = 30

\displaystyle n = 10 \sqrt[3]{2}

\displaystyle n = 20

\displaystyle n = 10

Correct answer:

\displaystyle n = 10 \sqrt[3]{3 }

Explanation:

The pyramid has a square base that is \displaystyle n units by \displaystyle n units, and its height is \displaystyle n units, as can be seen from this diagram,

Pyramid

The square base has area \displaystyle B = n^{2}; the pyramid has volume \displaystyle V = \frac{1}{3} \cdot n^{2} \cdot n = \frac{1}{3} n^{3}

Since the volume is 1,000, we can set this equal to 1,000 and solve for \displaystyle n:

\displaystyle \frac{1}{3} n^{3} = 1,000

\displaystyle n^{3} = 3,000

\displaystyle n = \sqrt[3]{3,000} = \sqrt[3]{1,000} \cdot \sqrt[3]{3 } = 10 \sqrt[3]{3 }

Example Question #3 : Pyramids

Find the volume of a pyramid with the following measurements:

  • length = 4in
  • width = 3in
  • height = 5in
Possible Answers:

\displaystyle 80\text{in}^3

\displaystyle 40\text{in}^3

\displaystyle 20\text{in}^3

\displaystyle 60\text{in}^3

\displaystyle 12\text{in}^3

Correct answer:

\displaystyle 20\text{in}^3

Explanation:

To find the volume of a pyramid, we will use the following formula:

\displaystyle V = \frac{l \cdot w \cdot h}{3}

where l is the length, w is the width, and h is the height of the pyramid.

 

Now, we know the base of the pyramid has a length of 4in.  We also know the base of the pyramid has a width of 3in.  We also know the pyramid has a height of 5in. 

Knowing this, we can substitute into the formula.  We get

\displaystyle V = \frac{4\text{in} \cdot 3\text{in} \cdot 5\text{in}}{3}

\displaystyle V = \frac{60\text{in}^3}{3}

\displaystyle V = 20\text{in}^3

Example Question #2 : How To Find The Volume Of A Pyramid

Find the volume of a pyramid with the following measurements:

  • length = 4cm
  • width = 9cm
  • height = 8cm
Possible Answers:

\displaystyle 24\text{cm}^3

\displaystyle 154\text{cm}^3

\displaystyle 288\text{cm}^3

\displaystyle 96\text{cm}^3

\displaystyle 126\text{cm}^3

Correct answer:

\displaystyle 96\text{cm}^3

Explanation:

To find the volume of a pyramid, we will use the following formula:

\displaystyle V = \frac{l \cdot w \cdot h}{3}

where l is the length, w is the width, and h is the height of the pyramid.

 

Now, we know the following measurements:

  • length = 4cm
  • width = 9cm
  • height = 8cm

Knowing this, we can substitute into the formula.  We get

\displaystyle V = \frac{4\text{cm} \cdot 9\text{cm} \cdot 8\text{cm}}{3}

\displaystyle V = \frac{288\text{cm}^3}{3}

\displaystyle V = 96\text{cm}^3

Example Question #2 : Pyramids

Find the volume of a pyramid with the following measurements:

  • length:  7in
  • width:  6in
  • height:  8in
Possible Answers:

\displaystyle 112\text{in}^3

\displaystyle 336\text{in}^3

\displaystyle 426\text{in}^3

\displaystyle 221\text{in}^3

\displaystyle 168\text{in}^3

Correct answer:

\displaystyle 112\text{in}^3

Explanation:

To find the volume of a pyramid, we will use the following formula:

\displaystyle V = \frac{lwh}{3}

where l is the length, w is the widthand h is the height of the pyramid.

Now, we know the following measurements:

  • length:  7in
  • width:  6in
  • height:  8in

So, we get

\displaystyle V = \frac{ 7\text{in} \cdot 6\text{in} \cdot 8\text{in}}{3}

\displaystyle V = \frac{42\text{in}^2 \cdot 8\text{in}}{3}

\displaystyle V = \frac{336\text{in}^3}{3}

\displaystyle V = 112\text{in}^3

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