Maths Olympiad Prep

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Number theory Difficulty 5.5 AIME, harder Prove it Romania

Find all triples of integers (x,y,z)(x, y, z) such that
x2+y2+z2=16(x+y+z). x^2 + y^2 + z^2 = 16(x + y + z).

Solution

Write the relation as (x8)2+(y8)2+(z8)2=192(x-8)^2 + (y-8)^2 + (z-8)^2 = 192 and recall that a square gives the remainder 00 or 11 when divided at 44 to infer that x8x-8, y8y-8, z8z-8 are all even. Set x8=2a1x-8 = 2a_1, y8=2b1y-8 = 2b_1, z8=2c1z-8 = 2c_1 to get a12+b12+c12=48a_1^2 + b_1^2 + c_1^2 = 48. Repeat the above argument to write a1=2a2a_1 = 2a_2, b1=2b2b_1 = 2b_2, c1=2c2c_1 = 2c_2 with a22+b22+c22=12a_2^2 + b_2^2 + c_2^2 = 12, then a2=2a3a_2 = 2a_3, b2=2b3b_2 = 2b_3, c2=2c3c_2 = 2c_3 with a32+b32+c32=3a_3^2 + b_3^2 + c_3^2 = 3.

The latter equality holds for a3,b3,c3=±1a_3, b_3, c_3 = \pm 1, that is whenever x8x-8, y8y-8 and z8z-8 are equal to 88 or 8-8. Consequently, the triples are (0,0,0)(0, 0, 0), (0,0,16)(0, 0, 16), (0,16,0)(0, 16, 0), (16,0,0)(16, 0, 0), (0,16,16)(0, 16, 16), (16,0,16)(16, 0, 16), (16,16,0)(16, 16, 0) and (16,16,16)(16, 16, 16).

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.