# Thread: Find the critical points.

1. ## Find the critical points.

f(x,y)=xy+lnx+2y^2

I know fx=y+(1/x) and fy=x+4y.

So my question is how do I find the critical points? I know it involves setting the two equations equal to 0, but I'm having a brain lapse. Please help.

2. As you said, set both equations equal to 0. Then take one of the equations and solve it for a variable. For instance, you could say (1.) $\displaystyle y=\frac{-1}{x}$. Then plug this in for y in the second equation and you get $\displaystyle 0=x-\frac{4}{x}$. Solve this equation for x and you get $\displaystyle x=\pm2$. Plugging in to (1.), you get $\displaystyle y(2)=\frac{-1}{2}$ and $\displaystyle y(-2)=\frac{1}{2}$.

When the system involves partial derivatives it seems to make people hesitate, but you can essentially just treat $\displaystyle f_{x}=0$ and $\displaystyle f_{y}=0$ like you would any other set of two equations involving two independent variables. It's just the interpretation of the results that makes the system special.

3. Thanks, this really cleared it up for me.