设f(x)为连续函数,证明∫0xf(t)(x-t)dt=∫0x(∫0tf(u)du)dt.

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问题 设f(x)为连续函数,证明∫0xf(t)(x-t)dt=∫0x(∫0tf(u)du)dt.

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答案利用分部积分法,有 ∫0x(∫0tf(u)du)dt=t∫0tf(u)du|0x-∫0xtf(t)dt =x∫0xf(t)dt-∫0xtf(t)dt=∫0xf(t)(x-t)dt 即∫0xf(t)(x-t)dt=∫0x(∫0tf(u)du)dt.

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