2021 SJI P2 Q9

2021 SJI P2 Q9

IB Year 5 | Grade 11
18 marks

The function \(\mathrm{f}\) is defined by \(\mathrm{f}(x)=\dfrac{12+16x-x^2}{x-6},\ x\neq6\).

  1. Express \(\mathrm{f}\) in the form \(\mathrm{f}(x)=A+Bx+\dfrac C{x-6}\), where \(A,\ B\) and \(C\) are constants to be determined.[4]
  2. Sketch the graph of \(y=\mathrm{f}(x)\), indicating clearly the coordinates of any turning points, axes intercepts and asymptotes.[5]
    1. Show that \(\mathrm{f}'(x)<0\) for all \(x\in\mathbb R,\ x\neq6\).
    2. Explain whether \(\mathrm{f}(x)\) is a decreasing function.[4]

Let \(\mathrm{g}(x)=kx\), where \(k\in\mathbb R\).

  1. Find the range of values of \(k\) for which the graphs of \(y=\mathrm{f}(x)\) and \(y=\mathrm{g}(x)\) do not intersect.[5]

Solution:

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Answer:(a) \(10-x+\dfrac{72}{x-6}\) (b) No turning points; intercepts \((0,-2),(8\pm2\sqrt{19},0)\); asymptotes \(x=6,y=10-x\). (c) Decreasing on each side of \(x=6\). (d) \(\dfrac{-10-2\sqrt6}3<k<\dfrac{-10+2\sqrt6}3\)

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