You want make use of the following theorems:
f^-1 (f(x)) = x
f(f^-1(x)) = x
# The theorem are fairly easy to prove if (x, y) is point of "f" than (y, x) is a point of f^-1 // By definition of inverse function
# Similarly if (x, y) is point of "f^-1" than (y, x) is a point of f
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