Using Fundamental Identities To Find Exact Values. Given certain trigonometric function values, we can find the other basic function values using reference angles on the unit circle. A second way to find other function values is using the fundamental identities.

 Example 1: Use identities to find the values of the other basic trigonometric functions given the following:

 Example 2: Use identities to find the values of the other basic trigonometric functions given the following: Since cos x is positive, x must be in quadrant I or quadrant IV. Since sin x is negative, x must be in quadrant III or quadrant IV. We conclude that x is in quadrant IV.