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.

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- Sep 13th 2012, 03:23 AMjacob93algebra subs with e^x
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. - Sep 13th 2012, 04:01 AMa tutorRe: algebra subs with e^x
You should not have two minuses in your third line.

- Sep 13th 2012, 04:10 AMjacob93Re: algebra subs with e^x
yes, since I have rewritten to e^minus, the inner derivative should be 1, not -1. Thanks.