用电脑拍的,有点不清楚.

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用电脑拍的,有点不清楚.

用电脑拍的,有点不清楚.

用电脑拍的,有点不清楚.

用电脑拍的,有点不清楚.
[S_{t:0->x}(x-t)f(t)dt]' = [xS_{t:0->x}f(t)dt - S_{t:0->x}tf(t)dt]'
= S_{t:0->x}f(t)dt + x[S_{t:0->x}f(t)dt]' - [S_{t:0->x}tf(t)dt]'
= S_{t:0->x}f(t)dt + xf(x) - xf(x)
= S_{t:0->x}f(t)dt