设f(x)=∫0xecostdt,求∫0πf(x)cosxdx.

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问题 设f(x)=∫0xecostdt,求∫0πf(x)cosxdx.

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答案0xf(x)cosxdx=∫0xf(x)d(sintx)=f(x)sinx|0π-∫0πf’(x)sinxdx =-∫0πf’(x)sinxdx=∫0πecosxsinxd(cosx)=∫0πecosxd(cosx) =ecosx|0π=e-e.

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