GRE Math : Isosceles Triangles

Study concepts, example questions & explanations for GRE Math

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Example Questions

Example Question #1 : Triangles

What is the perimeter of an isosceles triangle given that the sides 5 units long and half of the base measures to 4 units?

Possible Answers:
12
18
14
20
32
Correct answer: 18
Explanation:

The base of the triangle is 4 + 4 = 8 so the total perimeter is 5 + 5 + 8 = 18.

Example Question #1 : Isosceles Triangles

An acute Isosceles triangle has two sides with length \(\displaystyle a\) and one side length \(\displaystyle b\). The length of side \(\displaystyle \small a=\) \(\displaystyle \small \frac{3}{9}\) ft. If the length of \(\displaystyle \small b=\) half the length of side \(\displaystyle \small a\), what is the perimeter of the triangle? 

Possible Answers:

\(\displaystyle \small \frac{2}{6}\) foot

\(\displaystyle \small \frac{4}{6}\) foot

\(\displaystyle \small 10\) inches

\(\displaystyle \small 1\) foot

\(\displaystyle \small 6\) inches

Correct answer:

\(\displaystyle \small 10\) inches

Explanation:

This Isosceles triangle has two sides with a length of \(\displaystyle \small \frac{3}{9}\) foot and one side length that is half of the length of the two equivalent sides. 

To find the missing side, double the value of side \(\displaystyle \small a\)'s denominator:

\(\displaystyle \small \frac{3}{9}=\frac{1}{3}\). Thus, half of \(\displaystyle \small \frac{1}{3}=\frac{1}{6}\).

Therefore, this triangle has two sides with lengths of \(\displaystyle \small \frac{1}{3}\) and one side length of \(\displaystyle \small \frac{1}{6}\)

To find the perimeter, apply the formula: 

\(\displaystyle \small p=2a+b\)

\(\displaystyle \small p=\frac{1}{3}+\frac{1}{3}+\frac{1}{6}\)

\(\displaystyle \small p=\frac{2}{6}+\frac{2}{6}+\frac{1}{6}=\frac{5}{6}=\frac{10}{12}\) foot \(\displaystyle \small = 10\) inches


Example Question #52 : Geometry

An acute Isosceles triangle has two sides with length \(\displaystyle a\) and one side length \(\displaystyle b\). The length of side \(\displaystyle \small a=\) \(\displaystyle \small 13\). If the length of \(\displaystyle \small b=\) half the length of side \(\displaystyle \small a\), what is the perimeter of the triangle? 

Possible Answers:

\(\displaystyle \small 34\)

\(\displaystyle \small 26\)

\(\displaystyle \small 32.5\)

\(\displaystyle \small 34.5\)

\(\displaystyle \small 26.5\)

Correct answer:

\(\displaystyle \small 32.5\)

Explanation:

To solve this problem apply the formula: \(\displaystyle \small p=2a+b\).

However, first calculate the length of the missing side by: \(\displaystyle \small 13\div2=6.5\).

Thus, the solution is:

\(\displaystyle \small p=13+13+6.5=32.5\)

Example Question #2 : Triangles

Isos._cont._gre

Find the perimeter of the acute Isosceles triangle shown above. 

Possible Answers:

\(\displaystyle \small 120\)

\(\displaystyle \small 130\)

\(\displaystyle \small 132\)

\(\displaystyle \small 133\)

\(\displaystyle \small \small 60\)

Correct answer:

\(\displaystyle \small 132\)

Explanation:

To solve this problem apply the formula: \(\displaystyle \small p=2a+b\).

However, first calculate the length of the missing side by:

\(\displaystyle \small p=2(a)+12\)

\(\displaystyle \small a=12\times5=60\)

\(\displaystyle \small p=2(60)+12\)

\(\displaystyle \small p=120+12=132\)

Example Question #1 : How To Find The Perimeter Of An Acute / Obtuse Isosceles Triangle

An obtuse Isosceles triangle has two sides with length \(\displaystyle a\) and one side length \(\displaystyle b\). The length of side \(\displaystyle b=\) \(\displaystyle \frac{3}{4}\) ft. If the length of \(\displaystyle a=\) half the length of side \(\displaystyle b\), what is the perimeter of the triangle? 

Possible Answers:

\(\displaystyle 1\frac{1}{2}\) ft

\(\displaystyle 1\frac{1}{3}\) ft

\(\displaystyle 1\frac{2}{3}\) ft

\(\displaystyle \frac{12}{4}\) ft

Correct answer:

\(\displaystyle 1\frac{1}{2}\) ft

Explanation:

By definition, an Isosceles triangle must have two equivalent side lengths. Since we are told that \(\displaystyle b=\frac{3}{4}\) ft and that the sides with length \(\displaystyle a\) are half the length of side \(\displaystyle b\), find the length of \(\displaystyle a\) by: \(\displaystyle \frac{3}{4}=\frac{6}{8}\) and half of \(\displaystyle \frac{6}{8}=\frac{3}{8}\). Thus, both of sides with length \(\displaystyle a\) must equal \(\displaystyle \frac{3}{8}\) ft. 

Now, apply the formula: \(\displaystyle p=2a+b\).

\(\displaystyle p=\frac{3}{8}+\frac{3}{8}+\frac{6}{8}=\frac{12}{8}\)

Then, simplify the fraction/convert to mixed number fraction:


\(\displaystyle \frac{12}{8}=\frac{3}{2}=1\frac{1}{2}\)
 

Example Question #2 : Triangles

Isos._cont._gre

Find the perimeter of the acute Isosceles triangle shown above. 

Possible Answers:

\(\displaystyle \small 18\)

\(\displaystyle \small \frac{35}{3}\)

\(\displaystyle \small 21\)

\(\displaystyle \small 27\)

\(\displaystyle \small \frac{56}{3}\)

Correct answer:

\(\displaystyle \small 21\)

Explanation:

In order to solve this problem, first find the length of the missing sides. Then apply the formula: \(\displaystyle \small p=2a+b\)

Each of the missing sides equal: 

\(\displaystyle \small a=\frac{7}{3}\times4=\frac{28}{3}\) 

Then apply the perimeter formula:
\(\displaystyle \small p=\frac{28}{3}+\frac{28}{3}+\frac{7}{3}=\frac{63}{3}=21\)

Example Question #1 : How To Find The Perimeter Of An Acute / Obtuse Isosceles Triangle

Isos._gre_series_

An acute Isosceles triangle has two sides with length \(\displaystyle a\) and one side length \(\displaystyle b\). The length of side \(\displaystyle \small a=\) \(\displaystyle 8\) inches. If the length of \(\displaystyle \small b=\) \(\displaystyle \sqrt[3]{a}\), what is the perimeter of the triangle? 

Possible Answers:

\(\displaystyle 16\) inches

\(\displaystyle 24\) inches

\(\displaystyle 20\) inches

\(\displaystyle 18\) inches

Correct answer:

\(\displaystyle 18\) inches

Explanation:

In order to solve this problem, first find the length of the missing sides. Then apply the formula: \(\displaystyle \small p=2a+b\)

The missing side equals:

\(\displaystyle b=\sqrt[3]{a}=\sqrt[3]{8}=2\)

Then plug each side length into the perimeter formula:

\(\displaystyle p=2(8)+2=16+2=18\)

 

Example Question #62 : Geometry

Isos._gre_series_

An acute Isosceles triangle has two sides with length \(\displaystyle a\) and one side length \(\displaystyle b\). The length of side \(\displaystyle \small a=\) \(\displaystyle \sqrt{144}\) inches. If the length of \(\displaystyle \small b=a\times \frac{1}{2}\), what is the perimeter of the triangle?

Possible Answers:

\(\displaystyle \small 32\) inches

\(\displaystyle \small 18\) inches

\(\displaystyle \small 2\) feet

\(\displaystyle \small 2.7\) feet

\(\displaystyle \small 30\) inches

Correct answer:

\(\displaystyle \small 30\) inches

Explanation:

In order to solve this problem, first find the length of the missing sides. Then apply the formula: \(\displaystyle \small p=2a+b\)

The missing side equals:

\(\displaystyle \small a=\sqrt{144}=12\)

\(\displaystyle \small b=12\times \frac{1}{2}=\frac{12}{2}=6\)

Then, apply the perimeter formula by plugging in the side values: 

\(\displaystyle \small p=2(12)+6=24+6=30\)

Example Question #1 : Triangles

An acute Isosceles triangle has two sides with length \(\displaystyle a\) and one side length \(\displaystyle b\). The length of side \(\displaystyle \small a=\) \(\displaystyle \small \small \frac{3}{12}\) ft. If the length of  \(\displaystyle \small b=\small a\times \frac{1}{3}\) , what is the perimeter of the triangle? 

Possible Answers:

\(\displaystyle \small 7\) inches 

\(\displaystyle \small 6\) inches

\(\displaystyle \small \frac{1}{4}\) foot

\(\displaystyle \small \frac{3}{4}\) foot

\(\displaystyle \small \frac{9}{12}\) foot

Correct answer:

\(\displaystyle \small 7\) inches 

Explanation:

To solve this problem apply the formula: \(\displaystyle \small p=2a+b\).

However, first calculate the length of the missing side by:

\(\displaystyle \small \frac{3}{12}+\frac{3}{12}+(\frac{3}{12}\times\frac{1}{3})\) , Note that \(\displaystyle \small \frac{3}{36}=\frac{1}{12}\)


\(\displaystyle \small \small \frac{6}{12}+\frac{3}{36}=\frac{6}{12}+\frac{1}{12}=\frac{7}{12}\)

Since, it takes \(\displaystyle \small 12\) inches to make one foot, the perimeter is equal to \(\displaystyle \small 7\) inches. 

Example Question #64 : Geometry

An acute Isosceles triangle has two sides with length \(\displaystyle a\) and one side length \(\displaystyle b\). The length of side \(\displaystyle \small a=\) \(\displaystyle \small 8\) inches. The length of side \(\displaystyle \small b=\) \(\displaystyle \small a\times \frac{1}{4}\). Find the perimeter of the triangle.

Possible Answers:

\(\displaystyle \small 18\) inches

\(\displaystyle \small 19\) inches

\(\displaystyle \small 14\) inches

\(\displaystyle \small 13\) inches

\(\displaystyle \small 12\) inches

Correct answer:

\(\displaystyle \small 18\) inches

Explanation:

To find the perimeter of this triangle, apply the formula: 

\(\displaystyle \small p=2a+b\)

\(\displaystyle \small p=2(8)+(8\times \frac{1}{4})\)

\(\displaystyle \small p=16+\frac{8}{4}\)

\(\displaystyle \small p=16+2=18\)

Note: Since this is an acute Isosceles triangle, the length of the base must be smaller than the length of both of the equivalent sides. 

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