A relationship between sine and cosine

From the figure above the cosine of x can be defined as follows:

cos(x) = sin(90 - x)

This is because we know we have one 90 degree angle, a right angle, so the other two angles must equal 90 degrees, since the interior angles of a triangle always add up to 180 degrees. When the measure of two angles are dependent on each other in this way they can be called complements,. The name cosine originated this way, since it is the compliment of sine.