Originally Posted by

**cynlix** Hi, I'm having an issue using implicit differentiation. I don't think I'm doing it correctly on the first step.

Equation: "Find $\displaystyle dy/dx$ by implicit differentiation and evaluate the derivative at the given point"

$\displaystyle tan (x+y) =x , (0,0)$

What I have:

$\displaystyle

sec^2(x+y) (1+y') = (1)$

$\displaystyle

sec^2(x+y)+y'sec^2(x+y)

$

$\displaystyle

y'=\frac {1-sec^2(x+y)}{sec^2(x+y)}$

however, the answer for the first part is $\displaystyle -sin^2(x+y)$. Thanks in advance.