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How to Find the Missing Angle of a Triangle

Updated on April 5, 2012
Main Points About Triangles
Main Points About Triangles | Source

A Few Main Points You Need to Know About Triangles

#1 All angles within a triangle must always add to 180°

#2 The small square in the corner of a triangle denotes a 90° angle

#3 Arcs denote that two angles are congruent (meaning they have the exact same measure)

#4 A straight lines always measures 180°


Finding the Measure of a Triangle when you have the other Two Measures

Remember that the Triangle will always add to 180°

Let's assume that the measure of angle A is 65º and the measure of angle B is 70°.

So we are looking for the measure of angle C

The sum of angles A + B = 65 +70 = 135

Since the sum of all the angles must be 180 we subtract the other two angles from it.

180 - (A+B) = C

180 - 135 = 55

The measure of angles C is 55°


Finding Measures of Triangles when a Straight Line in involved

Remember that a straight line is 180°

In triangle #2 we have one angle and need to find the other two angles.

The first thing we are going to do is find angle A. Since angle A is part of a straight line we can conclude the following equation.

A + 85 = 180

A + 85 -85 = 180 -85

A = 95°

Now we have two of the angles that are part of the triangle

95 + 40 + B = 180

135 + B = 180

B = 180 - 135

B = 45°


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