设y=fn(x)+f(xn),其中f(x)具有二阶导数,求y”.

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问题 设y=fn(x)+f(xn),其中f(x)具有二阶导数,求y”.

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答案y’=nfn-1(x)f’(x)+f’(xn)·nxn-1 y”=n(n-1)fn-2(x)[f’(x)]2+nfn-1(x)f”(x)+ f”(xn)·nxn-1·nxn-1+f’(xn)·n(n-1)xn-2 =n(n-1)fn-2(x)[f’(x)]2+nfn-1(x)f”(x)+n2x2n-2f”(xn)+n(n-1)xn-2f’(xn)

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