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June 2023 p13 q7
755
The function \(f\) is defined by \(f(x) = 2 - \frac{5}{x+2}\) for \(x > -2\).
(a) State the range of \(f\).
(b) Obtain an expression for \(f^{-1}(x)\) and state the domain of \(f^{-1}\).
Solution
(a) The function \(f(x) = 2 - \frac{5}{x+2}\) has a horizontal asymptote at \(y = 2\). As \(x \to \infty\), \(f(x) \to 2\). Therefore, the range of \(f\) is \(y < 2\).
(b) To find the inverse, start with \(y = 2 - \frac{5}{x+2}\).
Rearrange to solve for \(x\):
\(y(x+2) = 2(x+2) - 5\)
\(yx + 2y = 2x + 4 - 5\)
\(yx + 2y = 2x - 1\)
\(2y + 1 = 2x - yx\)
\(2y + 1 = x(2 - y)\)
\(x = \frac{2y+1}{2-y}\)
Thus, \(f^{-1}(x) = \frac{2x+1}{2-x}\).
The domain of \(f^{-1}\) is determined by the range of \(f\), which is \(x < 2\).