# Angle Measurement

### Angle Measurement

Angle Measurement − Step-by-step instructions for how to measure angles on the math part of the ACT test.

#### Rules for Angle Measurement

The sum of all three angles in any triangle must be equal to 180 degrees.

An isosceles triangle has two equal sides and two equal angles.

An equilateral triangle has three equal sides and three equal angles.

Angles that have the same measurement in degrees are called congruent angles.

Equilateral triangles are sometimes called congruent triangles.

Now try the following sample exam questions.

Question 1:

Side A of an equilateral triangle is 3 units long. What is the individual length of each of the two other sides?

A. 9 units

B. 6 units

C. 3 units

D. 1.5 units

E. Cannot be determined from the information provided.

As stated above, an equilateral triangle has three equal sides and three equal angles.

So, if one side of an equilateral triangle is 3 units long, the other sides must also be 3 units long.

Question 2:

Consider the isosceles triangle in the diagram below:

What is the measurement of ∠B ?

A. 36°

B. 45°

C. 72°

D. 144°

E. Cannot be determined from the information provided.

Remember these concepts:

The sum of all three angles in a triangle is always 180°.

Two sides of an isosceles triangle are equal in length, and their corresponding angles are also equal.

So, we need to deduct the degrees given from 180° to find out the total degrees of the two other angles:

180° − 36° = 144°

Now divide this result by two in order to find out how many degrees each angle has:

144° ÷ 2 = 72°

Go back to the Geometry Problems Page.