# Math Help - algebra subs with e^x

1. ## algebra subs with e^x

Why can't I make the substitution
$-e^{-(x+y)} \cdot -1 = e^{-(x+y)}$?

(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.

2. ## Re: algebra subs with e^x

You should not have two minuses in your third line.

3. ## Re: algebra subs with e^x

yes, since I have rewritten to e^minus, the inner derivative should be 1, not -1. Thanks.