Why can't I make the substitution
$\displaystyle -e^{-(x+y)} \cdot -1 = e^{-(x+y)}$?
(See calculation here https://dl.dropbox.com/u/586465/Phot...%2017%2040.jpg I had to find ∂f/∂x=0 and I thought x=-1 but the book says x=1.
Why can't I make the substitution
$\displaystyle -e^{-(x+y)} \cdot -1 = e^{-(x+y)}$?
(See calculation here https://dl.dropbox.com/u/586465/Phot...%2017%2040.jpg I had to find ∂f/∂x=0 and I thought x=-1 but the book says x=1.