设f(x)在区间[0,a]上严格递增且连续,f(0)=0,g(x)为f(x)的反函数,试证成立等式:∫0af(x)dx=∫0f(a)[a-g(x)]dx.

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问题 设f(x)在区间[0,a]上严格递增且连续,f(0)=0,g(x)为f(x)的反函数,试证成立等式:∫0af(x)dx=∫0f(a)[a-g(x)]dx.

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答案设y=f(x),则x=g(y),注意到f(0)=0,故 ∫0af(x)dx=∫f(0)f(a)yd(g(y))=yg(y)|f(0)f(a)-∫f(0)f(a)g(y)dy =f(a)g(f(a))-∫0f(a)g(y)dy=af(a)-∫0f(a)g(x)dx =∫0f(a)[a-g(x)]dx.

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