Intermediate Geometry : How to find the volume of a prism

Study concepts, example questions & explanations for Intermediate Geometry

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Example Questions

Example Question #94 : Prisms

Pyramid

The above figure shows a square pyramid inscribed inside a cube. The pyramid has volume 100. Give the volume of the cube.

Possible Answers:

\(\displaystyle 200\)

\(\displaystyle 300\)

\(\displaystyle 233\frac{1}{3}\)

\(\displaystyle 333\frac{1}{3}\)

\(\displaystyle 250\)

Correct answer:

\(\displaystyle 300\)

Explanation:

Let \(\displaystyle s\) be the length of one side of the cube. Then the base of the pyramid is a square with sidelength \(\displaystyle s\), and its area is 

\(\displaystyle B = s^{2}\).

The volume of a pyramid is one third the product of its height and the area of its base. The height of the pyramid is \(\displaystyle s\), so the volume is

\(\displaystyle V_{1} = \frac{1}{3} Bh = \frac{1}{3} s^{2} \cdot s = \frac{1}{3} s^{3}\)

The volume of the cube is \(\displaystyle V_{2} = s^{3}\), so

\(\displaystyle V_{1} = \frac{1}{3} V_{2}\)

or, equivalently,

\(\displaystyle 3 \cdot V_{1} =3 \cdot \frac{1}{3} V_{2}\)

\(\displaystyle V_{2} = 3 \cdot V_{1}\)

That is, the volume of the cube is three times that of the pyramid, and, since the pyramid has volume 100, the volume of the cube is \(\displaystyle 3 \cdot 100 =300\).

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

A rectangular box has two sides with the following lengths: 

\(\displaystyle 3$ cm$\) and \(\displaystyle 4 $ cm$\)

If it possesses a volume of \(\displaystyle 84$ cm$^{3}\), what is the area of its largest side?

Possible Answers:

12

21

16

49

28

Correct answer:

28

Explanation:

The volume of a rectangular prism is found using the following formula:

\(\displaystyle V=l\times w\times h\)

If we substitute our known values, then we can solve for the missing side.

\(\displaystyle 84=3\times 4\times x\)

\(\displaystyle 84=12x\)

Divide both sides of the equation by 12.

\(\displaystyle \frac{84}{12}=\frac{12x}{12}\)

\(\displaystyle x=7\)

We now know that the missing length equals 7 centimeters.

This means that the box can have sides with the following dimensions: 3cm by 4cm; 7cm by 3cm; or 7cm by 4cm. The greatest area of one side belongs to the one that is 7cm by 4cm. 

\(\displaystyle A=l\times w\)

\(\displaystyle A=4\times 7\)

\(\displaystyle A=28\)

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