Make sure your function is one-to-one. The inverse of a function f(x) (which is written as f-1 (x))is essentially the reverse: put in your y value, and you'll get your initial x value back. Be sure © Elizabeth Stapel 2000-2011 All Rights Reserved. The inverse of a function f(x) (which is written as f-1 (x))is essentially the reverse: put in your y value, and you'll get your initial x value back. When you make that change, you call the new f (x) by its true name — f–1 (x) — and solve for this function. Inverse Function = what z-score corresponds to a known area/probability? Not all functions are naturally “lucky” to have inverse functions. If a function is not one-to-one, you will need to apply domain restrictions so that the part of the function you are using is one-to-one. Find a local math tutor,