根据换元积分法或者分部积分法求定积分 50
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∫(0->2π) x^2.cosx dx
=∫(0->2π) x^2 dsinx
=[x^2.sinx]|(0->2π) -2∫(0->2π) xsinx dx
=0 +2∫(0->2π) xdcosx
=2[xcosx]|(0->2π) -2∫(0->2π) cosx dx
=4π - 2[sinx]|(0->2π)
=4π
=∫(0->2π) x^2 dsinx
=[x^2.sinx]|(0->2π) -2∫(0->2π) xsinx dx
=0 +2∫(0->2π) xdcosx
=2[xcosx]|(0->2π) -2∫(0->2π) cosx dx
=4π - 2[sinx]|(0->2π)
=4π
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