PSAT Math : How to find the volume of a tetrahedron

Study concepts, example questions & explanations for PSAT Math

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Example Questions

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

Tetra_2

Note: Figure NOT drawn to scale.

The above triangular pyramid has volume 25. To the nearest tenth, evaluate \displaystyle h.

Possible Answers:

\displaystyle h \approx 7.2

\displaystyle h \approx 10.8

\displaystyle h \approx 8.8

\displaystyle h \approx 13.3

Insufficient information is given to answer the problem.

Correct answer:

\displaystyle h \approx 10.8

Explanation:

We are looking for the height of the pyramid.

The base is an equilateral triangle with sidelength 4, so its area can be calculated as follows:

\displaystyle B = \frac{s^{2} \sqrt{3} }{4}

\displaystyle B = \frac{4^{2} \sqrt{3} }{4}

\displaystyle B = \frac{16 \sqrt{3} }{4}

\displaystyle B = 4\sqrt{3}

The height \displaystyle h of a pyramid can be calculated using the fomula

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

We set \displaystyle V = 25 and \displaystyle B = 4\sqrt{3} and solve for \displaystyle h:

\displaystyle \frac{1}{3} \cdot 4\sqrt{3} \cdot h = 25

\displaystyle h = \frac{25 \cdot 3}{4 \sqrt{3}} = \frac{75}{4 \sqrt{3}} \approx \frac{75}{4 \cdot 1.7321} \approx 10.8

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

Tetra_1

Note: Figure NOT drawn to scale.

Give the volume (nearest tenth) of the above triangular pyramid.

Possible Answers:

\displaystyle 55.4

\displaystyle 27.7

\displaystyle 18.5

\displaystyle 22.6

\displaystyle 15.1

Correct answer:

\displaystyle 18.5

Explanation:

The height of the pyramid is \displaystyle h = 8. The base is an equilateral triangle with sidelength 4, so its area can be calculated as follows:

\displaystyle B = \frac{s^{2} \sqrt{3} }{4}

\displaystyle B = \frac{4^{2} \sqrt{3} }{4}

\displaystyle B = \frac{16 \sqrt{3} }{4}

\displaystyle B = 4\sqrt{3}

The volume of a pyramid can be calculated using the fomula

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

\displaystyle V = \frac{1}{3} \cdot 4\sqrt{3} \cdot 8 = \frac{32\sqrt{3} }{3} \approx \frac{32 \cdot 1.7321}{3} \approx 18.5

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

A regular tetrahedron has an edge length of \displaystyle 4. What is its volume?

Possible Answers:

\displaystyle 3

\displaystyle \frac{64}{3\sqrt{2}}

\displaystyle \frac{32}{3\sqrt{2}}

\displaystyle \frac{32}{\sqrt{2}}

\displaystyle \frac{32}{3}

Correct answer:

\displaystyle \frac{32}{3\sqrt{2}}

Explanation:

The volume of a tetrahedron is found with the equation \displaystyle V=\frac{a^3}{6\sqrt{2}}, where \displaystyle a represents the length of an edge of the tetrahedron.

Plug in 4 for the edge length and reduce as much as possible to find the answer:

 

\displaystyle V=\frac{4^3}{6\sqrt{2}}

\displaystyle V=\frac{64}{6\sqrt{2}}

\displaystyle V=\frac{32}{3\sqrt{2}}

The volume of the tetrahedron is \displaystyle \frac{32}{3\sqrt{2}}.

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