Triangle Angle Sum Theorem

Interior Angles of a Triangle

Drag vertices to explore. Use the slider to prove they sum to 180°.

Triangle Mode

∠A + ∠B + ∠C = 180°

Linear Proof

Angle A 0°

Angle B 0°

Angle C 0°

Drag corners to reshape the triangle

Geometrical Fact: The sum of the interior angles of any Euclidean triangle is equal to a straight angle (180°).