A triangle is a three-sided polygon. It has three interior angles that add up to 180° (a + b + c = 180°). An exterior angle of a triangle is equal to the sum of the two interior angles that are opposite (d = b + c). The **perimeter** of a triangle is equal to the sum of the lengths of its three sides, the **height** of a triangle is equal to the length from the **base** to the opposite **vertex** (angle) and the **area** equals one-half triangle base x height: a = ½ base x height.

A triangle has three interior angles that add up to __________°.