Missing Angles & the Triangle Angle Sum (Grades 6–8)
The three angles inside any triangle always add up to 180°. So if you know two angles, you can find the third by subtracting from 180. Likewise, angles on a straight line add to 180°, and angles around a point add to 360° — these facts let you fill in missing angles step by step.
Understanding missing angles & the triangle angle sum
The three angles inside any triangle always add up to 180°. So if you know two angles, you can find the third by subtracting from 180. Likewise, angles on a straight line add to 180°, and angles around a point add to 360° — these facts let you fill in missing angles step by step.
Key Idea
The three angles inside any triangle always add up to 180°. So if you know two angles, you can find the third by subtracting from 180. Likewise, angles on a straight line add to 180°, and angles around a point add to 360° — these facts let you fill in missing angles step by step.
Seeing it in action
Worked example
A triangle has angles 50° and 60°. Find the third.
Third = 180 − (50 + 60) = 180 − 110 = 70°.
Triangle angles add to 180°.
Try a few
Triangle angles 90° and 45° — third?
Triangle angles 30° and 30° — third?
Two angles on a straight line: one is 110°, the other?
Angle Outpost
A calm geometry game for finding missing angles on a line, around a point, and inside a triangle.
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