The function \(f\) is defined by \(f(x) = 2 - \frac{3}{4x - p}\) for \(x > \frac{p}{4}\), where \(p\) is a constant.
(b) Express \(f^{-1}(x)\) in the form \(\frac{p}{a} - \frac{b}{cx - d}\), where \(a, b, c\) and \(d\) are integers.
(c) Hence state the value of \(p\) for which \(f^{-1}(x) \equiv f(x)\).
Solution
To find \(f^{-1}(x)\), start by setting \(y = f(x) = 2 - \frac{3}{4x - p}\).
Rearrange to solve for \(x\):
\(y - 2 = -\frac{3}{4x - p}\)
\((y - 2)(4x - p) = -3\)
\(4xy - 8x = py - 2p - 3\)
\(4x(y - 2) = p(y - 2) - 3\)
\(4x = \frac{p(y - 2) - 3}{y - 2}\)
\(x = \frac{p(y - 2) - 3}{4(y - 2)}\)
\(x = \frac{p}{4} - \frac{3}{4(y - 2)}\)
Thus, \(f^{-1}(x) = \frac{p}{4} - \frac{3}{4x - 8}\).
For \(f^{-1}(x) \equiv f(x)\), equate the expressions:
\(2 - \frac{3}{4x - p} = \frac{p}{4} - \frac{3}{4x - 8}\)
Equating the coefficients gives \(p = 8\).
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