凑微分得∫xdx/(1-x^2) =(1/2)×d(x^2)/(1-x^2) =-(1/2)×d(-x^2)/(1-x^2) =-(1/2)×d(1-x^2)/(1-x^2) =-(1/2)ln|1-x^2)|+C(其中C为任意常数）
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