Let f(x) = x2 – 81. Find f–1(x)?

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Oddman answered
We can solve for x
  f(x) = x^2 - 81
  f(x) + 81 = x^2
  √(f(x) + 81) = x

Your inverse function is
  f-1(x) = √(x+81)
We have shown the positive branch of the square root. The inverse relation is actually double-valued, as x may be either sign and still give the same result for f(x).

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