If x + y + z = 2 1 9 6
3 x + y + z = 2 0 7 6
x + 3 y + z = 1 8 6 0
x + y + 3 z = 4 8 0
and if x , y and z are positive integers,
Find x 3 2 + y 3 2 + z 3 2
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Since 3 x + y + z , x + 3 y + z and x + y + 3 z give us integer answer.
⇒ Let 3 x = a , 3 y = b and 3 z = c , we can rewrite the question to
a 3 + b 3 + c 3 = 2 1 9 6 --------(1)
a + b 3 + c 3 = 2 0 7 6 -------(2)
a 3 + b + c 3 = 1 8 6 0 -------(3)
a 3 + b 3 + c = 4 8 0 -------(4)
(1) - (2) , we get a 3 − a = 1 2 0
Factorizes both size of the equation, a ( a − 1 ) ( a + 1 ) = 2 3 × 3 × 5 a ( a + 1 ) ( a − 1 ) = 4 × 5 × 6 a = 5
We can find the value of b and c by the same method.
(1) - (3) , we get b 3 − b = 3 3 6
Factorizes both size of the equation, b ( b − 1 ) ( b + 1 ) = 2 4 × 3 × 7 b ( b + 1 ) ( b − 1 ) = 6 × 7 × 8 b = 7
(1) - (4) , we get c 3 − c = 1 7 1 6
Factorizes both size of the equation, c ( c − 1 ) ( c + 1 ) = 2 2 × 3 × 1 1 × 1 3 c ( c + 1 ) ( c − 1 ) = 1 1 × 1 2 × 1 3 c = 1 2
Therefore, x 3 2 + y 3 2 + z 3 2 = a 2 + b 2 + c 2 = 5 2 + 7 2 + 1 2 2 = 2 5 + 4 9 + 1 4 4 = 2 1 8