How do you find f^-1(x)f1(x) given f(x)=(x+2)^2+6f(x)=(x+2)2+6?

1 Answer
Jun 13, 2017

f(x)f(x) has no inverse function, but...

Explanation:

Given:

f(x) = (x+2)^2+6f(x)=(x+2)2+6

We can attempt to derive an inverse function as follows:

Let:

y = f(x) = (x+2)^2+6y=f(x)=(x+2)2+6

Subtract 66 from both ends to get:

y-6 = (x+2)^2y6=(x+2)2

Take the square root of both sides, allowing for both possible signs:

+-sqrt(y-6) = x+2±y6=x+2

Transpose and subtract 22 from both sides to get:

x = -2+-sqrt(y-6)x=2±y6

Note that for any value of yy (apart from 66) in the range [6, oo)[6,) of f(x)f(x), this formula gives us two possible values of xx.

Since f(x)f(x) is many to one on its domain (-oo, oo)(,), its inverse is not a function, but a relation. So we cannot define f^(-1)(y)f1(y) unless we restrict the range.

Alternatively, we can define an inverse image function F^(-1)(y)F1(y) from [6, oo)[6,) to 2^RR (the set of subsets of RR) by:

F^(-1)(y) = { -2+sqrt(y-6), -2-sqrt(y-6) }