Lagrange multiplier problem - function of two variables with one constraint

[abery]

Homework Statement

Find the maximum and minimum values of f(x,y) = 2x^2+4y^2 - 4xy -4x
on the circle defined by x^2+y^2 = 16.

Homework Equations

Lagrange's method, where f_x = lambda*g_x, f_y= lambda*g_y (where f is the given function and g(x,y) is the circle on which we are looking for the extrema)

The Attempt at a Solution

Computed the partials, and was able to end up with an equation like 2y -2 = \lambda*(x-y)

From this, critical points look like they might be (1, root 15) and (root 15, 1) but this does not seem to be the answer.

Any help at all is much appreciated!

 

[HallsofIvy]

Well, it would have helped if you had show precisely what you did, not just generalities. "end up with an equation like 2y -2 = λ *(x-y)". Actually, I think you have f and g reversed but it really doesn't matter whether you use f_x= \lambda g_x or g_x=(1/\lambda) f_x

f(x,y)= 2x^2+ 4y^2- 4xy- 4x and g(x,y)= x^2+ y^2= 16.
so f_x= 4x- 4y- 4 and g_x= 2x. Your first equation is 4x- 4y- 4= 2\lambda x. f_y= 8y- 4x and g_y= 2y. Your second equation is 8y- 4x= 2\lambda y. Those, together with the condition that x^2+ y^2= 16, give you three equations to solve for x, y, and \lambda.

But the value of \lambda is not really necessary to solve this problem and I find that it is often best to eliminate \lambda from the first two equations by dividing one by the other.

 

[abery]

Thanks for your help, HallsOfIvy. My problem is with solving the resulting system of equations. (Thanks for the suggestion to divide the equations)

After doing that and rearranging the first two, I get: x² - xy -y² - y =0

And using the third (original) constraint, I get -2y² - xy - y - 16 = 0, but am not sure how to proceed.

Well, it would have helped if you had show precisely what you did, not just generalities. "end up with an equation like 2y -2 = λ *(x-y)". Actually, I think you have f and g reversed but it really doesn't matter whether you use f_x= \lambda g_x or g_x=(1/\lambda) f_x

f(x,y)= 2x^2+ 4y^2- 4xy- 4x and g(x,y)= x^2+ y^2= 16.
so f_x= 4x- 4y- 4 and g_x= 2x. Your first equation is 4x- 4y- 4= 2\lambda x. f_y= 8y- 4x and g_y= 2y. Your second equation is 8y- 4x= 2\lambda y. Those, together with the condition that x^2+ y^2= 16, give you three equations to solve for x, y, and \lambda.

But the value of \lambda is not really necessary to solve this problem and I find that it is often best to eliminate \lambda from the first two equations by dividing one by the other.