Actually, the proper terminology is a bijective function but I will not go into that. (If the function is countinous and one-to-one then it has an inverse).Originally Posted byaussiekid90

Yes, dreadfully true it is that the trigonometric functions have no inverses because they are not one to one. However, what we can do is restrict the domain. We define the (not reciprocal but rather inverse) as that , notice that this is the inverse on this interval.

Checking the requirements for the interval we see that,

.

Similar things are done to the other 5 trigonomteric functions, however, some domains are restricted differently than others. For example, the domain of is