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How to do it faster?

  1. Apr 2, 2005 #1
    [tex] 2\pi \int {x^3 \sin 2x\,dx} \Rightarrow \left\{ \begin{array}{l}
    u = 2x \\
    du = 2dx \\
    \end{array} \right\} \Rightarrow \frac{\pi }{8}\int {u^3 \sin u\,du} = \frac{\pi }{8}\left( { - u^3 \cos u + \int {u^2 \cos u\,du} } \right) = \frac{\pi }{8}\left( { - u^3 \cos u + u^2 \sin u - \int {u\sin u\,du} } \right) = \frac{\pi }{8}\left( { - u^3 \cos u + u^2 \sin u + u\cos u - \sin u} \right) = \frac{{\pi \left[ { - 8x^3 \cos \left( {2x} \right) + 4x^2 \sin \left( {2x} \right) + 2x\cos x - \sin x} \right]}}{8} [\tex]

    How can I do this faster? Are there things I can skip or connect--etc--?
     
  2. jcsd
  3. Apr 4, 2005 #2

    dextercioby

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    3 times part integration would do it...

    [tex]\int \left( x^3\sin 2x\right) dx=\allowbreak -\frac 12x^3\cos 2x+\frac 34x^2\sin 2x-\frac 38\sin 2x+\frac 34x\cos 2x +C [/tex]

    Daniel.
     
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