Two angles are called complementary when their measures add to 90 degrees. Two angles are called supplementary when their measures add up to 180 degrees. They don't have to be next to each other, just so long as the total is 90 degrees. Chapter 25 discusses and uses the concept of trigonometric ratios of complementary angles along with their applications.
Examples:
• 60° and 30° are complementary angles.
• 5° and 85° are complementary angles.
Since the sum of complementary angles equals 90 degrees, therefore if we know the measure of one angle, then we can find the unknown angle easily.
For example, if one of the two angles is 45 degrees, then;
x + 45 = 90
x = 90 – 45 = 45°