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Number theory Difficulty 5.0 AIME Prove it Slovenia

Find all integral solutions xx and yy of the equation
3xy+2x+y=12. 3xy + 2x + y = 12.

Solution

Rewrite the equation as x(3y+2)=12yx(3y+2) = 12 - y. Obviously, 3y+203y+2 \neq 0 divides 12y12 - y, so 3y+23y+2 divides 3(12y)+(3y+2)=383(12 - y) + (3y+2) = 38. Since 3y+23y+2 gives the remainder of 22 when divided by 33, there are four possibilities. The number 3y+23y+2 is equal to 19,1,2-19, -1, 2 or 3838, so yy is equal to 7,1,0-7, -1, 0 or 1212 and xx is equal to 1,13,6-1, -13, 6 or 00 respectively. The solutions are (1,7),(13,1),(6,0)(-1, -7), (-13, -1), (6, 0) and (0,12)(0, 12).

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