# Thread: find dy/dx

1. ## find dy/dx

• find $\frac{dy}{dx}$ if $ln(xy)=x+y$

this is what l tried to do to solve it:
1. $ln(xy)=x+y$
2. $lnx+lny=x+y$
3. $y=lnx+lny-x$
4. $y'=\frac{1}{x}+\frac{1}{y}-1$
5. $y'=\frac{y+x}{xy}-1$
6. $y'=\frac{y+x-xy}{xy}$

however, the answer is suppose to be $\frac{xy-y}{x-xy}$

how do you solve this?

2. Originally Posted by algebra2
3. $y=lnx+lny-x$
4. $y'=\frac{1}{x}+\color{red}\frac{1}{y}\color{black}-1$
No. Remember that you are differentiating with respect to $x,$ so you must use the chain rule: $\frac d{dx}[\ln y]=\frac1y\frac{dy}{dx}.$

For this problem, just use implicit differentiation, as follows.

$\ln xy=x+y$

$\Rightarrow\frac d{dx}\left[\ln xy\right]=\frac d{dx}\left[x+y\right]$

$\Rightarrow\frac d{dx}\left[\ln x+\ln y\right]=1+\frac{dy}{dx}$

$\Rightarrow\frac1x+\frac{dy/dx}y=1+\frac{dy}{dx}$

$\Rightarrow\frac{dy/dx}y-\frac{dy}{dx}=1-\frac1x$

$\Rightarrow\frac{dy}{dx}\left(\frac1y-1\right)=\frac{x-1}x$

$\Rightarrow\frac{dy}{dx}=\frac{x-1}x\cdot\frac y{1-y}$

$\Rightarrow\frac{dy}{dx}=\frac{xy-y}{x-xy}$