Differentiating this implicit expression? I am given:
$$\tfrac{1}{4}(x+y)^2 + \tfrac{1}{9}(x - y)^2 = 1$$
Using the chain rule, and factoring out $y'$, I'm left with:
$$y' \left(\tfrac{1}{2}(x+y) - \tfrac{2}{9}(x-y)\right) = 0$$
Now I need to isolate $y'$ but I'm not sure how. 
Should I do:
$$y' = \frac{1}{\tfrac{1}{2}(x+y) - \tfrac{2}{9}(x-y)}$$
Am I going about this question the correct way?
Thanks

Your first step is wrong. It should be $$\frac{1}{2}(1+y')(x+y)+\frac{2}{9}(1-y')(x-y)=0$$
