Understanding missing angles & the triangle angle sum
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°.
A common mistake, and the fix
Subtracting from the wrong total.
A triangle’s angles add to 180°, so with 50° and 60° the third is 180 − 110 = 70°; subtracting from 360 gives 250°, bigger than a straight line. It is angles all the way around a point that add to 360°.
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.
Try it in Numeris.
A short lesson, then practice with feedback on every answer.
Put the reasoning on paper.
Try six fraction, angle, perimeter and area questions with explained answers, for grades 4–6.
Free printable · 3 pagesText version (screen-reader friendly): Draw it. Explain it. Math sample
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