y'=-xe^(-x^2/2)
y''=-e^(-x^2/2)+x^2*e^(-x^2/2)=(x^2-1)e^(-x^2/2)
=(x+1)(x-1)e^(-x^2/2)
e^(-x^2/2)>0,
则当x0,函数下凸,
当-1