A triangle is a polygon with three sides.
The interior angles in a triangle add up to .
There are several ways to calculate the area of a triangle.
Choose one side as the “base” and measure the “height” of the third vertex above this base. Then the area is .
Let and be the lengths of two of the sides and the angle between them. Then the area is .
Let be the semi-perimeter (that is, half the perimeter). And let , , be the side lengths. Then the area is .