The angle addition postulates states that if an angle UVW has a point S lying in its interior, then the sum of angle UVS and angle SVW must equal angle UVW, or ?UVS + ?SVW= ?UVW.
One can show this graphically by drawing the angle ?UVW, where point V denotes the vertex angle. The point S lies inside the angle. By drawing a line from point V through point S, one divides the angle ?UVW into two angles ?UVS and ?SVW. When added together, these two angles must be equal to the original angle ?UVW.
If the angle UVW is a 90 degree angle, then the two angles ?UVS and ?SVW are complementary angles. However, when these two angles add up to 180 degrees, they are supplementary angles.