Maths Olympiad Prep

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Problem 923

AMC 12 late, AIME early
Number theory Difficulty 4.7 Prove it Macedonian Mathematical Competitions · North Macedonia

Find all triples of positive integers (x,y,z)(x, y, z) such that
xyz+xy+yz+zx+x+y+z=243. xyz + xy + yz + zx + x + y + z = 243.

This one wants a proof. Work it on paper, then read the official solution and mark yourself. Be honest about it: the record is only any use to you if it is.

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Official solution

We add 11 on both sides of the given equality and we get
xyz+xy+yz+zx+x+y+z+1=244 xyz + xy + yz + zx + x + y + z + 1 = 244
or
xy(z+1)+x(z+1)+y(z+1)+z+1=244 xy(z+1) + x(z+1) + y(z+1) + z + 1 = 244
or
(z+1)(xy+x+y+1)=244. (z+1)(xy + x + y + 1) = 244.
From where we obtain
(x+1)(y+1)(z+1)=244. (x+1)(y+1)(z+1) = 244.
Because 244=2261244 = 2 \cdot 2 \cdot 61 we get that (x+1,y+1,z+1)(x+1, y+1, z+1) is one of the following triples (2,2,61)(2, 2, 61), (2,61,2)(2, 61, 2), (61,2,2)(61, 2, 2). Finally, the desired triples are (1,1,60)(1, 1, 60), (1,60,1)(1, 60, 1), (60,1,1)(60, 1, 1).

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