For some reason I make a mistake with the minus-sign, I don't see where. We substitute:
u = x^2
du/dx = 2x
dx = du/2x
Thus we can write the integral as:
Using integration by parts yields:
-u * e^(-u) - (-1*e^(-u)) = (1-u)*e^(-u)
But my calculator says -(1+u) - what am I doing wrong?