Inverse and composite of linear functions, a line from its intercepts and the reflection in y equals x

Inverse and composite of linear functions, a line from its intercepts and the reflection in y equals x

14 marks

The function \(\mathrm{f}\) is defined by \(\mathrm{f}(x)=3x-4\), \(x\in\mathbb{R}\).

  1. Find
    1. \(\mathrm{f}(5)\)
    2. \(\mathrm{f}^{-1}(x)\).[3]

The diagram shows part of the straight-line graph of a function \(\mathrm{h}\). The line crosses the \(x\)-axis at \(x=-3\) and the \(y\)-axis at \(y=2\).

  1. Write down \(\mathrm{h}(x)\) in terms of \(x\).[2]
  2. Solve \(\mathrm{h}^{-1}(x)=3\).[2]

The function \(\mathrm{g}\) is defined by \(\mathrm{g}(x)=mx+c\), \(x\in\mathbb{R}\), where \(m\) and \(c\) are rational constants.

  1. Given that \(\mathrm{h}(x)=(\mathrm{f}^{-1}\circ\mathrm{g})(x)\) for all \(x\), find \(m\) and \(c\).[4]

The function \(\mathrm{k}\) satisfies \(\mathrm{h}(\mathrm{k}(x))=x\) for all \(x\).

  1. Find \(\mathrm{k}(x)\) in terms of \(x\).[2]
  2. State the single geometric transformation which takes the graph of \(y=\mathrm{k}(x)\) to the graph of \(y=\mathrm{h}(x)\).[1]

Solution:

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Answer:(a)(i) \(11\) (a)(ii) \(\mathrm{f}^{-1}(x)=\dfrac{x+4}{3}\) (b) \(\mathrm{h}(x)=\dfrac23x+2\) (c) \(x=4\) (d) \(m=2,\ c=2\) (e) \(\mathrm{k}(x)=\dfrac32x-3\) (f) reflection in the line \(y=x\)

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