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Inverse of f(x)=x^2-8x+12?

f(x) = x^2 - 8x + 12

y = x^2 -8x +12

Lets make RHS as perfect sqaure

y = (x - 4)^2 -16 +12

y = (x -4)^2 - 4

Now plug x=y and y =x and solve for y

x = (y -4)^2 - 4

(x+4) = ( y -4)^2

y - 4= +/- sqrt(x+4)

y = +/- sqrt(x +4) +4

f^-1(x) = 4 - /+ sqrt( x+4)