Triangles have several properties we must remember:
- The interior angles of a triangle always add up to exactly two right angles \( 180^\circ \).
- If we extend one of the sides of a triangle at a given vertex, the angle formed with the other side at that vertex is called an exterior angle, and is equal to the sum of the opposite two angles in the triangle.
- The base angles of an isosceles triangle are equal.
- The area of a triangle is equal to \( 1 / 2 \) the base length times the height.