Quadratic sine derivative pattern

Quadratic sine derivative pattern

IB Year 6 | Grade 12
13 marks
TGM Original Questions

Let \(\mathrm{f}(x)=x^2\sin x\).

  1. Using the definition \(\mathrm{g}'(x)=\lim_{h\to0}\dfrac{\mathrm{g}(x+h)-\mathrm{g}(x)}{h}\), show from first principles that the derivative of \(\mathrm{g}(x)=x^2\) is \(2x\).[4]
  2. Prove by induction that, for every positive integer \(n\),
    \[\mathrm{f}^{(n)}(x)=[x^2-n(n-1)]\sin\left(x+\frac{n\pi}{2}\right)-2nx\cos\left(x+\frac{n\pi}{2}\right).\][9]

Solution:

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Answer:(a) \(\mathrm{g}'(x)=2x\) (b) \(\mathrm{f}^{(n)}(x)=[x^2-n(n-1)]\sin(x+n\pi/2)-2nx\cos(x+n\pi/2)\), \(n\ge1\)

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