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Rsarette
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Homework Statement
Say that P is a point on the surface xyz=8. Is it true or false that you can always find another point Q on the surface such that Q is further away from the origin than P is?
Homework Equations
∇f(x,y,z)=λ∇g(x,y,z) where ∇g=0
The Attempt at a Solution
Let f(x,y,z)=x2+y2+z2
and g(x,y,z)=xyz-8
Then the ∇f(x,y,z)=λ
2x=λ
2y=λ
2z=λ
Therefore x=y=z
and then plugging into xyz=8
x3=8
x=2
But then the point (2,2,2) is only the square root of 12 away from the origin, where a point like (8,1,1) is the square root of 66 away from the origin. So I think I minimized the distance instead of maximizing the distance fro the origin, but how do I maximize the distance?