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1992 Paper 1 Q3
D: 1500.0 B: 1486.1

Evaluate

  1. \({\displaystyle \int_{-\pi}^{\pi}\left|\sin x\right|\,\mathrm{d}x,}\)
  2. \({\displaystyle \int_{-\pi}^{\pi}\sin\left|x\right|\,\mathrm{d}x},\)
  3. \({\displaystyle \int_{-\pi}^{\pi}x\sin x\,\mathrm{d}x},\)
  4. \({\displaystyle \int_{-\pi}^{\pi}x^{10}\sin x\,\mathrm{d}x.}\)


Solution:

  1. \begin{align*} \int_{-\pi}^\pi |\sin x | \d x &= \int_{-\pi}^{0} - \sin x \d x + \int_0^\pi \sin x \d x \\ &= \left [\cos x \right]_{-\pi}^{0} +[-\cos x]_0^{\pi} \\ &= 1-(-1)+(1)-(-1) \\ &= 4 \end{align*}
  2. \begin{align*} \int_{-\pi}^\pi \sin | x | \d x &= \int_{-\pi}^0 - \sin x \d x + \int_0^\pi \sin x \d x \\ &= 4 \end{align*}
  3. \begin{align*} \int_{-\pi}^\pi x \sin x \d x &= \left [ -x \cos x \right]_{-\pi}^\pi + \int_{-\pi}^{\pi} \cos x \d x \\ &= \pi -(-\pi) + \left [\sin x \right]_{-\pi}^\pi \\ &= 2\pi \end{align*}
  4. \begin{align*} \int_{-\pi}^{\pi} x^{10} \sin x \d x &\underbrace{=}_{x^{10}\sin x \text{ is odd}} 0 \end{align*}