Trigonometric functions arise frequently in problems, and often we are interested in finding specific angle measures. For instance, you may want to find some angle {{MATH_75}} such that {{MATH_76}} Hence we want to be able to ``undo'' trigonometric functions. However, since trigonometric functions are not one-to-one, meaning there are are infinitely many angles with {{MATH_77}}, it is impossible to find a true inverse function for {{MATH_78}}. Nevertheless, it is useful to have something like an inverse to these functions, however imperfect. The usual approach is to pick out some collection of angles that produce all possible values exactly once. If we ``discard'' all other angles, the resulting function has a proper inverse. In other words, we are restricting the domain of the trigonometric function in order to find an inverse. The function {{MATH_79}} takes on all values between {{MATH_80}} and {{MATH_81}} exactly once on the interval {{MATH_82}}.