Intermediate Geometry : How to find an angle in an acute / obtuse isosceles triangle

Study concepts, example questions & explanations for Intermediate Geometry

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Example Questions

Example Question #1 : How To Find An Angle In An Acute / Obtuse Isosceles Triangle

An isoceles triangle has a vertex angle that is twenty more than twice the base angle.  What is the difference between the vertex and base angles?

Possible Answers:

\(\displaystyle 20\)

\(\displaystyle 60\)

\(\displaystyle 100\)

\(\displaystyle 40\)

\(\displaystyle 150\)

Correct answer:

\(\displaystyle 60\)

Explanation:

A triangle has \(\displaystyle 180\) degrees.  An isoceles triangle has one vertex angle and two congruent base angles.

Let \(\displaystyle x\) = the base angle and \(\displaystyle 2x\ +\ 20\) = vertex angle

So the equation to solve becomes \(\displaystyle x\ +\ x\ +\ 2x\ +\ 20 = 180\)

or

 \(\displaystyle 4x\ +\ 20=180\)

so the base angle is \(\displaystyle 40\) and the vertex angle is \(\displaystyle 100\) and the difference is \(\displaystyle 60\).

Example Question #1 : How To Find An Angle In An Acute / Obtuse Isosceles Triangle

An ssosceles triangle has interior angles of \(\displaystyle 32\) degrees and \(\displaystyle 74\) degrees. Find the missing angle. 

Possible Answers:

\(\displaystyle 37^\circ\) 

\(\displaystyle 90^\circ\) 

\(\displaystyle 74^\circ\) 

\(\displaystyle 15^\circ\)

\(\displaystyle 32^\circ\) 

Correct answer:

\(\displaystyle 74^\circ\) 

Explanation:

Isosceles triangles always have two equivalent interior angles, and all three interior angles of any triangle always have a sum of \(\displaystyle 180\) degrees. 

Thus, the solution is:

\(\displaystyle 32+74=106\)

\(\displaystyle 180-106=74\)

\(\displaystyle Check: 74+74+32=180\)

Example Question #2 : How To Find An Angle In An Acute / Obtuse Isosceles Triangle

The largest angle in an obtuse isosceles triangle is \(\displaystyle 98\) degrees. Find the measurement of one of the two equivalent interior angles. 

Possible Answers:

\(\displaystyle 42^\circ\) 

\(\displaystyle 41^\circ\) 

\(\displaystyle 82^\circ\) 

\(\displaystyle 2^\circ\)

\(\displaystyle 90^\circ\) 

Correct answer:

\(\displaystyle 41^\circ\) 

Explanation:

Isosceles triangles always have two equivalent interior angles, and all three interior angles of any triangle always have a sum of \(\displaystyle 180\) degrees. Since this is an obtuse isosceles triangle, the two missing angles must be acute angles.  

Thus, the solution is:

\(\displaystyle 180-98=82\)

\(\displaystyle 82\div2=41\)

Example Question #3 : How To Find An Angle In An Acute / Obtuse Isosceles Triangle

The two equivalent interior angles of an obtuse isosceles triangle each have a measurement of \(\displaystyle 28\) degrees. Find the measurement of the obtuse angle. 

Possible Answers:

\(\displaystyle 99^\circ\)

\(\displaystyle 112^\circ\) 

\(\displaystyle 124^\circ\) 

\(\displaystyle 56^\circ\) 

\(\displaystyle 134^\circ\) 

Correct answer:

\(\displaystyle 124^\circ\) 

Explanation:

Isosceles triangles always have two equivalent interior angles, and all three interior angles of any triangle always have a sum of \(\displaystyle 180\) degrees.

Thus, the solution is:

\(\displaystyle 28+28=56\)

\(\displaystyle 180-56=124\)

Example Question #1 : How To Find An Angle In An Acute / Obtuse Isosceles Triangle

In an obtuse isosceles triangle the angle measurements are, \(\displaystyle x^\circ\)\(\displaystyle x^\circ\), and \(\displaystyle (10x-2)=128^\circ\). Find the measurement of one of the acute angles. 

Possible Answers:

\(\displaystyle 4^\circ\) 

\(\displaystyle 26^\circ\) 

\(\displaystyle 13^\circ\) 

\(\displaystyle 10^\circ\) 

\(\displaystyle 32^\circ\) 

Correct answer:

\(\displaystyle 13^\circ\) 

Explanation:

Isosceles triangles always have two equivalent interior angles, and all three interior angles of any triangle always have a sum of \(\displaystyle 180\) degrees. Since this is an obtuse isosceles triangle, the two missing angles must be acute angles. 

The solution is:

\(\displaystyle 180-154=26\)

However, \(\displaystyle 26\) degrees is the measurement of both of the acute angles combined.

Each individual angle is \(\displaystyle 26\div2=13\).

Example Question #5 : How To Find An Angle In An Acute / Obtuse Isosceles Triangle

In an acute isosceles triangle the two equivalent interior angles each have a measurement of \(\displaystyle 53\) degrees. Find the missing angle. 

Possible Answers:

\(\displaystyle 53^\circ\) 

\(\displaystyle 75^\circ\) 

\(\displaystyle 62^\circ\) 

\(\displaystyle 42^\circ\)

\(\displaystyle 74^\circ\) 

Correct answer:

\(\displaystyle 74^\circ\) 

Explanation:

Isosceles triangles always have two equivalent interior angles, and all three interior angles of any triangle always have a sum of \(\displaystyle 180\) degrees. Since this is an acute isosceles triangle, all of the interior angles must be acute angles.

The solution is:

\(\displaystyle 53+53=106\)

\(\displaystyle 180-106=74\) 

Example Question #6 : How To Find An Angle In An Acute / Obtuse Isosceles Triangle

In an acute isosceles triangle the two equivalent interior angles are each \(\displaystyle 55\) degrees. Find the missing angle. 

Possible Answers:

\(\displaystyle 78^\circ\) 

\(\displaystyle 13^\circ\)

\(\displaystyle 70^\circ\) 

\(\displaystyle 62^\circ\) 

\(\displaystyle 140^\circ\) 

Correct answer:

\(\displaystyle 70^\circ\) 

Explanation:

Isosceles triangles always have two equivalent interior angles, and all three interior angles of any triangle always have a sum of \(\displaystyle 180\) degrees. Since this is an acute isosceles triangle, all of the interior angles must be acute angles.

The solution is:

\(\displaystyle 55+55=110\)

\(\displaystyle 180-110=70\)

Example Question #7 : How To Find An Angle In An Acute / Obtuse Isosceles Triangle

The largest angle in an obtuse isosceles triangle is \(\displaystyle 138\) degrees. Find the measurement of one of the equivalent interior angles. 

Possible Answers:

\(\displaystyle 23^\circ\) 

\(\displaystyle 44^\circ\) 

\(\displaystyle 15^\circ\) 

\(\displaystyle 21^\circ\) 

\(\displaystyle 42^\circ\) 

Correct answer:

\(\displaystyle 21^\circ\) 

Explanation:

Isosceles triangles always have two equivalent interior angles, and all three interior angles of any triangle always have a sum of \(\displaystyle 180\) degrees. Since this is an obtuse Isosceles triangle, the two missing angles must be acute angles.  

Thus, the solution is:

\(\displaystyle 180-138=42\)

\(\displaystyle 42\div2=21\)

Example Question #41 : Acute / Obtuse Isosceles Triangles

In an obtuse isosceles triangle the largest angle is \(\displaystyle 118\) degrees. Find the measurement of one of the acute angles. 

Possible Answers:

\(\displaystyle 15^\circ\) 

\(\displaystyle 62^\circ\) 

\(\displaystyle 31^\circ\) 

\(\displaystyle 18^\circ\) 

\(\displaystyle 64^\circ\) 

Correct answer:

\(\displaystyle 31^\circ\) 

Explanation:

Isosceles triangles always have two equivalent interior angles, and all three interior angles of any triangle always have a sum of \(\displaystyle 180\) degrees. Since this is an obtuse isosceles triangle, the two missing angles must be acute angles. 

The solution is:

\(\displaystyle 180-118=62\)

\(\displaystyle 62\div2=31\)

Example Question #41 : Acute / Obtuse Isosceles Triangles

In an acute isosceles triangle the measurement of the non-equivalent interior angle is \(\displaystyle 86\) degrees. Find the measurement of one of the equivalent interior angles. 

Possible Answers:

\(\displaystyle 52^\circ\) 

\(\displaystyle 94^\circ\)

\(\displaystyle 45^\circ\) 

\(\displaystyle 14^\circ\)

\(\displaystyle 47^\circ\) 

Correct answer:

\(\displaystyle 47^\circ\) 

Explanation:

Isosceles triangles always have two equivalent interior angles, and all three interior angles of any triangle always have a sum of \(\displaystyle 180\) degrees. Since this is an acute isosceles triangle, all of the interior angles must be acute angles.

The solution is:

\(\displaystyle 180-86=94\)

\(\displaystyle 94\div2=47\)

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