y = xe^x
dy/dx = xe^x + e^x = e^x (x + 1)
dy/dx = 0
e^x (x + 1) = 0
x + 1 = 0 或 (e^x = 0,舍)
x = -1
最小值是当x = -1时,y = (-1)e^(-1) = -1/e