求二阶导数可以判断凹凸区间；二阶导数为零且在两侧异号的点,即是拐点.f'(x)=e^[(-1/2)x²)](-x),f''(x)=e^[(-1/2)x²)](-x)²- e^[(-1/2)x²)]=(x²-1)e^[(-1/2)x²)],令　f''(x)>0,得x
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