Find the set of points of the surface x y z = 1 which are nearest to the origin.
Input the sum of the squares of their coordinates.
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The distance from the surface to the origin is given by x 2 + y 2 + z 2 . By AM-GM inequality, we have:
x 2 + y 2 + z 2 ≥ 3 3 ( x y z ) 2 = 3
Equality occurs when x 2 = y 2 = z 2 = 1 . With x y z = 1 , there are two possibilities, all 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 ) , and ( 1 , − 1 , − 1 ) . Therefore, the sum of squares of all coordinates is 4 × 3 = 1 2 .
Your inequality is wrong
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Using Lagrange multipliers: 2 x = λ y z , 2 y = λ x z , 2 z = λ x y ⇒ x 2 = 2 λ x y z , y 2 = 2 λ x y z , z 2 = 2 λ x y z ⇒ x 2 = y 2 = z 2 = ( 2 λ ) 2 . So, y = ± x and z = ± x . This leads to x 3 = ± 1 . So, x = 1 when y = z = ± 1 , and x = − 1 when y = − z = ± 1 . Putting this together, we get the points (1, 1, 1), (1, -1, -1), (-1, 1, -1), (-1, -1, 1).