ISEE Upper Level Math : How to find the length of an edge of a tetrahedron

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

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Example Questions

Example Question #1 : Tetrahedrons

A triangular pyramid, or tetrahedron, with volume 100 has four vertices with Cartesian coordinates 

\(\displaystyle (0,0,0), (n,0, 0), (0,10,0), (0, 0, 5)\)

where \(\displaystyle n > 0\).

Evaluate \(\displaystyle n\)

Possible Answers:

\(\displaystyle n = 18\)

\(\displaystyle n = 12\)

\(\displaystyle n = 9\)

\(\displaystyle n = 24\)

\(\displaystyle n = 6\)

Correct answer:

\(\displaystyle n = 12\)

Explanation:

The tetrahedron is as follows (figure not to scale):

Tetrahedron

This is a triangular pyramid with a right triangle with legs 10 and \(\displaystyle n\) as its base; the area of the base is 

 \(\displaystyle B = \frac{1}{2} \cdot 10 \cdot n = 5n\)

The height of the pyramid is 5, so

\(\displaystyle V = \frac{1}{3} \cdot 5n \cdot 5 = \frac{25n}{3}\)

Set this equal to 100 to get \(\displaystyle n\):

\(\displaystyle \frac{25n}{3} = 100\)

\(\displaystyle n = 100 \cdot \frac{3}{25} = 12\)

 

Example Question #1 : Tetrahedrons

A triangular pyramid, or tetrahedron, with volume 1,000 has four vertices with Cartesian coordinates 

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

where \(\displaystyle n > 0\).

Evaluate \(\displaystyle n\)

Possible Answers:

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

\(\displaystyle n = 10 \sqrt[3]{6}\)

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

\(\displaystyle n = 10\)

\(\displaystyle n = 10 \sqrt[3]{12}\)

Correct answer:

\(\displaystyle n = 10 \sqrt[3]{6}\)

Explanation:

The tetrahedron is as follows:

Pyramid

This is a triangular pyramid with a right triangle with two legs of measure \(\displaystyle n\) as its base; the area of the base is 

 \(\displaystyle B = \frac{1}{2} \cdot n \cdot n = \frac{n^{2}}{2}\)

Since the height of the pyramid is also \(\displaystyle n\), the volume is

\(\displaystyle V = \frac{1}{3} \cdot \frac{n^{2}}{2} \cdot n = \frac{n^{3}}{6}\).

Set this equal to 1,000:

\(\displaystyle \frac{n^{3}}{6} = 1,000\)

\(\displaystyle n^{3} = 6,000\)

\(\displaystyle n = \sqrt[3]{6,000} = \sqrt[3]{1,000} \cdot \sqrt[3]{6} = 10 \sqrt[3]{6}\)

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

A triangular pyramid, or tetrahedron, with volume 240 has four vertices with Cartesian coordinates 

\(\displaystyle (0,0,0), (n,0, 0), (0,n,0), (0, 0, 24)\)

where \(\displaystyle n > 0\).

Evaluate \(\displaystyle n\)

Possible Answers:

\(\displaystyle n = 2 \sqrt{30}\)

\(\displaystyle n = \sqrt{30}\)

\(\displaystyle n = 2 \sqrt{15}\)

\(\displaystyle n= 6\sqrt{10}\)

\(\displaystyle n= 3\sqrt{10}\)

Correct answer:

\(\displaystyle n = 2 \sqrt{15}\)

Explanation:

The tetrahedron is as follows (figure not to scale):

Tetrahedron

This is a triangular pyramid with a right triangle with two legs of measure \(\displaystyle n\) as its base; the area of the base is 

 \(\displaystyle B = \frac{1}{2} \cdot n \cdot n = \frac{n^{2}}{2}\)

The height of the pyramid is 24, so the volume is

\(\displaystyle V = \frac{1}{3} \cdot \frac{n^{2}}{2} \cdot 24= 4 n^{2}\)

Set this equal to 240 to get \(\displaystyle n\):

\(\displaystyle 4 n^{2} = 240\)

\(\displaystyle n^{2} = 60\)

\(\displaystyle n = \sqrt{60} = \sqrt{4} \cdot \sqrt{15} = 2 \sqrt{15}\)

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