18: Solve the equation y = f (x) for x. In order to be able to cancel a trig function with its inverse, the argument must be from the basic interval where the trig function is invertible. Draw a picture and introduce x₀ = x − kT for a suitable integer k.

The easiest way to prove that f is 1-1 is through monotonicity. Find derivative of f and investigate its sign on M.

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