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Nov 2019 p13 q2
756
The function \(g\) is defined by \(g(x) = x^2 - 6x + 7\) for \(x > 4\). By first completing the square, find an expression for \(g^{-1}(x)\) and state the domain of \(g^{-1}\).
Solution
First, complete the square for \(g(x) = x^2 - 6x + 7\).
Rewrite \(x^2 - 6x + 7\) as \((x-3)^2 - 9 + 7\).
This simplifies to \((x-3)^2 - 2\).
Set \(y = (x-3)^2 - 2\) and solve for \(x\).
\(y + 2 = (x-3)^2\)
\(x - 3 = \pm \sqrt{y + 2}\)
Since \(x > 4\), choose the positive root: \(x = 3 + \sqrt{y + 2}\).
Thus, \(g^{-1}(x) = 3 + \sqrt{x + 2}\).
The domain of \(g^{-1}\) is \(x > -1\) because \(x + 2 > 0\).