# Implicit exponential differentiation?

1. Jan 13, 2012

### jrjack

1. The problem statement, all variables and given/known data

Find an equation of the tangent line to the curve xe^y+ye^x=1 at point (0,1).

2. Relevant equations

I do not recall seeing the Implicit Function Theroem before, I even went back in my book (Stewart Calculus 6th) to check. I found this post but it does not help me understand the steps involved: https://www.physicsforums.com/showthread.php?t=79275

3. The attempt at a solution

$$xe^{y}+ye^{x}=1$$
$$\frac{d}{dx}(xe^{y})+\frac{d}{dx}(ye^{x})=\frac{d}{dx}(1)$$
I am not sure if this requires the product rule but,
$$\frac{d}{dx}(xe^{y})=e^{y}+xe^{y}$$
Then, I'm not sure on the other part???
$$\frac{d}{dx}(ye^{x})=y'e^{x}+ye^{x}$$

2. Jan 13, 2012

### Dick

Don't forget the chain rule.
$$\frac{d}{dx}(e^{y})=e^{y} y'$$.

3. Jan 13, 2012

### Bacle2

Yes, the point is that if fx(x,y) and/or fy(x,y) are not 0 in a 'hood of (0,1), where y=f(x), or x=f(y) , depending on whether fx or fy is non-zero. The standard example ( I know of) , is that of the sphere.

4. Jan 13, 2012

### timacho

why can't i use product rule?

5. Jan 13, 2012

### Dick

Both parts require the product rule. So, of course you need it. You will need the chain rule too.

6. Jan 13, 2012

### jrjack

Yes, I now realize I need to use both rules.

7. Jan 13, 2012

### jrjack

So,
$$\frac{d}{dx}(ye^{x})=ye^{x}+y'e^{x}$$
giving me,
$$xe^{y}y'+e^y+ye^{x}+e^{x}y'=0$$
$$y'(xe^{y}+e^{x})=-e^{y}-ye^{x}$$
$$y'=\frac{-e^{y}-ye^{x}}{xe^{y}+e^{x}}$$
at point (0,1).
slope=(-e-1)

8. Jan 13, 2012

### Dick

Seems ok to me.

9. Jan 13, 2012