反常（广义）积分 xe^(-x^2) 范围是0到正无穷 =∫-1/2e^(-x^2)d(-x^2)=-1/2e^(-x^2)(下标O,上标+无穷大）=-1/2(1/e)^x^2(下标0,上标+无穷大）=0+1/2=1/2