### Video Summary:

In a right triangle ABC, with right angle C, the acute angles A and B are complementary.
Since angles A and B are complementary, and
sin A = cos B, the functions sine and cosine are called cofunctions. Similarly for secant and cosecant, and tangent and cotangent. A reference angle for an angle , written , is the positive acute angle made by the terminal side of angle and the x-axis.