Call Lagrange

Calculus Level 5

Find the set of points of the surface x y z = 1 xyz=1 which are nearest to the origin.

Input the sum of the squares of their coordinates.


The answer is 12.

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2 solutions

James Wilson
Aug 21, 2017

Using Lagrange multipliers: 2 x = λ y z , 2 y = λ x z , 2 z = λ x y x 2 = λ 2 x y z , y 2 = λ 2 x y z , z 2 = λ 2 x y z x 2 = y 2 = z 2 = ( λ 2 ) 2 2x=\lambda yz, 2y=\lambda xz, 2z=\lambda xy \Rightarrow x^2=\frac{\lambda}{2}xyz, y^2=\frac{\lambda}{2}xyz, z^2=\frac{\lambda}{2}xyz \Rightarrow x^2=y^2=z^2=(\frac{\lambda}{2})^2 . So, y = ± x y=\pm x and z = ± x z=\pm x . This leads to x 3 = ± 1 x^3=\pm 1 . So, x = 1 x=1 when y = z = ± 1 y=z=\pm 1 , and x = 1 x=-1 when y = z = ± 1 y=-z=\pm 1 . Putting this together, we get the points (1, 1, 1), (1, -1, -1), (-1, 1, -1), (-1, -1, 1).

Chew-Seong Cheong
Jul 18, 2017

The distance from the surface to the origin is given by x 2 + y 2 + z 2 \sqrt{x^2+y^2+z^2} . By AM-GM inequality, we have:

x 2 + y 2 + z 2 3 ( x y z ) 2 3 = 3 \begin{aligned} x^2 + y^2 + z^2 & \ge 3\sqrt[3]{(xyz)^2} = 3 \end{aligned}

Equality occurs when x 2 = y 2 = z 2 = 1 x^2=y^2=z^2=1 . With x y z = 1 xyz=1 , there are two possibilities, all x , y , z = 1 x, y, z =1 or one of them is 1 and the other two are -1. Altogether 4 points ( 1 , 1 , 1 ) (1,1,1) , ( 1 , 1 , 1 ) (-1,-1,1) , ( 1 , 1 1 ) (-1,1-1) , and ( 1 , 1 , 1 ) (1,-1,-1) . Therefore, the sum of squares of all coordinates is 4 × 3 = 12 4\times 3=\boxed{12} .

Your inequality is wrong

Reynan Henry - 3 years, 8 months ago

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Thanks, I have changed it.

Chew-Seong Cheong - 3 years, 8 months ago

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