You can do it using the custom node. Look into the asin(x), acos(x) and atan2(x) functions. Those should return the inverse sine, cosine and tangent in radians.
I suggest you bookmark this page as sooner or later you may need many of these:
That said the inverse functions can be expensive. For what do you need them? There is often a cheaper way to come up with the same information using different math that runs faster on the GPU.