- #1
glid02
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Hey, I need help with a problem involving Lagrange multipliers...
Here is the question:
Find the absolute maximum and minimum of the function
f(x,y) = x^2-y^2 subject to the constraint x^2+y^2=289.
As usual, ignore unneeded answer blanks, and list points in lexicographic order.
I found that the max is 289 and the min is -289.
I took the gradients of each and came up with:
2x-2y = lambda2x+lambda2y
x=lambda(x)
-y=lambda(y)
Here's where I'm stuck, I know lambda = +/-1, but I can't come up with an equation to solve for x or y in terms of y or x. The best idea I can come up with is to solve for lambda from -y=lambda(y), which is just
-y/y and plug it into x=lambda(x), but that doesn't help any when solving for y.
I guess lambda = 1 when y = 0 and lambda = -1 when x = 0.
OK, I found the correct answers, but I could still use an explanation as to how to list them lexicographically.
The correct answer is:
max = 289 at points (-17, 0) and (17,0)
min = -289 at points (0,-17) and (0,17)
When y = 0, lambda = 1 so (lambda(x))^2+0=289
which is x^2=289, x = 17
When x = 0, lambda = -1 so (lambda(y))^2+0=289
which is -y^2=289, y = -17
So why do both the lexicographically first points contain negative coordinates?
I just rambled a whole lot. Basically if someone could give me an explanation for why the correct answer is correct that'd be great.
Thanks a lot,
Gregg
Here is the question:
Find the absolute maximum and minimum of the function
f(x,y) = x^2-y^2 subject to the constraint x^2+y^2=289.
As usual, ignore unneeded answer blanks, and list points in lexicographic order.
I found that the max is 289 and the min is -289.
I took the gradients of each and came up with:
2x-2y = lambda2x+lambda2y
x=lambda(x)
-y=lambda(y)
Here's where I'm stuck, I know lambda = +/-1, but I can't come up with an equation to solve for x or y in terms of y or x. The best idea I can come up with is to solve for lambda from -y=lambda(y), which is just
-y/y and plug it into x=lambda(x), but that doesn't help any when solving for y.
I guess lambda = 1 when y = 0 and lambda = -1 when x = 0.
OK, I found the correct answers, but I could still use an explanation as to how to list them lexicographically.
The correct answer is:
max = 289 at points (-17, 0) and (17,0)
min = -289 at points (0,-17) and (0,17)
When y = 0, lambda = 1 so (lambda(x))^2+0=289
which is x^2=289, x = 17
When x = 0, lambda = -1 so (lambda(y))^2+0=289
which is -y^2=289, y = -17
So why do both the lexicographically first points contain negative coordinates?
I just rambled a whole lot. Basically if someone could give me an explanation for why the correct answer is correct that'd be great.
Thanks a lot,
Gregg