PSAT Math : Tetrahedrons

Study concepts, example questions & explanations for PSAT Math

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Example Questions

Example Question #1 : How To Find The Length Of An Edge Of A Tetrahedron

Tetra_1

Refer to the above tetrahedron, or four-faced solid. The surface area of the tetrahedron is 444. Evaluate \displaystyle s to the nearest tenth. 

Possible Answers:

\displaystyle s \approx 256.3

\displaystyle s \approx 314.0

\displaystyle s \approx 17.8

\displaystyle s \approx 21.1

\displaystyle s \approx 16.0

Correct answer:

\displaystyle s \approx 16.0

Explanation:

The tetrahedron has four faces, each of which is an equilateral triangle with sidelength \displaystyle s. Since the total surface area is 444, each triangle has area one fourth of this, or 111. To find \displaystyle s, set \displaystyle A = 111 in the formula for the area of an equilateral triangle:

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

\displaystyle \frac{s^{2} \sqrt{3} }{4} =111

\displaystyle s^{2} \sqrt{3} =444

\displaystyle s^{2}=\frac{444}{ \sqrt{3} } \approx \frac{444}{ 1.732 } \approx 256.34

\displaystyle s\approx \sqrt{256.34} \approx 16.0

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}}.

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

A regular tetrahedron has four congruent faces, each of which is an equilateral triangle. 

A given tetrahedron has edges of length six inches. Give the total surface area of the tetrahedron.

Possible Answers:

\displaystyle 9 \sqrt{3}\textrm{ in}^{2}

\displaystyle 36 \textrm{ in}^{2}

\displaystyle 9 \sqrt{2}\textrm{ in}^{2}

\displaystyle 36 \sqrt{2}\textrm{ in}^{2}

\displaystyle 36 \sqrt{3}\textrm{ in}^{2}

Correct answer:

\displaystyle 36 \sqrt{3}\textrm{ in}^{2}

Explanation:

The area of an equilateral triangle is given by the formula

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

Since there are four equilateral triangles that comprise the surface of the tetrahedron, the total surface area is 

\displaystyle SA = 4 A = 4 \cdot \frac{s^{2}\sqrt{3}}{4} = s^{2}\sqrt{3}

Substitute \displaystyle s = 6:

\displaystyle SA = s^{2}\sqrt{3} = 6^{2}\sqrt{3} = 36\sqrt{3} square inches.

Example Question #1 : Tetrahedrons

Tetra_1

Give the surface area of the above tetrahedron, or four-faced solid, to the nearest tenth.

Possible Answers:

Insufficient information is given to answer the question.

\displaystyle 114.3

\displaystyle 98.0

\displaystyle 84.9

\displaystyle 69.3

Correct answer:

\displaystyle 84.9

Explanation:

The tetrahedron has four faces, each of which is an equilateral triangle with sidelength 7. Each face has area

\displaystyle A = \frac{s^{2} \sqrt{3} }{4} = \frac{7^{2} \sqrt{3} }{4} = \frac{49 \sqrt{3} }{4} \approx \frac{49 \cdot 1.7321}{4} \approx 21.22

The total surface area is four times this, or about \displaystyle 4 \cdot 21.22 = 84.88.

Rounded, this is 84.9.

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