Thread: Local Max/Min Values using Partial Derivatives

f(x, y) = 2x^3 + xy^2 + 5x^2 + y^2 + 8

Find the local max, min and saddle points of the function.

So far, I have:
-partial derivative of x= 6x^2 + y^2 + 10x
-partial derivative of y= 2xy+2y

But I'm really confused what to do next. My book says to set both of the partial derivative equations to 0 and then solve for one of the variables, but I get some weird answer that doesn't work in both equations.

Thanks so much for your help!

Originally Posted by juicysharpie

f(x, y) = 2x^3 + xy^2 + 5x^2 + y^2 + 8

Find the local max, min and saddle points of the function.

So far, I have:
-partial derivative of x= 6x^2 + y^2 + 10x
-partial derivative of y= 2xy+2y

But I'm really confused what to do next. My book says to set both of the partial derivative equations to 0 and then solve for one of the variables, but I get some weird answer that doesn't work in both equations.

Thanks so much for your help!

So you need to solve this system of equaitons

and



Notice that we can factor the bottom equaiton to get

so by the zero factor principle we get

either or

So now if we can plug that into the top equation to get si we get two possible ordered pairs

You need to use the other solution for x above to find the rest of your critical points. I hope this helps

The first step is as the book says and as TheEmptySet started you on. Once you have all the critical points you're not done! You have to show whether they are local max/min or saddle point.
Call .
Now, if and , then is a local minimum.
If and then is a local maximum.
If then is a saddle point.

Finally, if and then you cannot conclude using this formula, and I don't really know how to proceed. I hope your exercise doesn't fall into this category.