题目
8.设f(x)=(x^2027)/(x-1),则f^(2027)(x)=____.
8.设$f(x)=\frac{x^{2027}}{x-1}$,则$f^{(2027)}(x)=$____.
题目解答
答案
令 $ y = x - 1 $,则 $ f(x) = \frac{(y+1)^{2027}}{y} $。展开得
$(y+1)^{2027} = \sum_{k=0}^{2027} \binom{2027}{k} y^k,$
故
$f(x) = \sum_{k=0}^{2027} \binom{2027}{k} y^{k-1} = \frac{1}{y} + \sum_{k=1}^{2027} \binom{2027}{k} y^{k-1}.$
对 $ f(x) $ 求 2027 阶导数,仅 $ \frac{1}{y} $ 项贡献非零结果,其余多项式项导数为 0。
$\left( \frac{1}{y} \right)^{(2027)} = \left( \frac{1}{x-1} \right)^{(2027)} = -\frac{2027!}{(x-1)^{2028}}.$
答案: $\boxed{-\frac{2027!}{(x-1)^{2028}}}$