+2729 If $a$ and $b$ are positive integers for which $ab - 3a + 4b = 131$, what is the minimal possible value of $|a - b|$?
+135 We can use Simon's Favourite Factoring Trick to solve this problem.
ab−3a+4b=131
a(b−3)+4b=131
a(b−3)+4b−12=131−12
a(b−3)+4(b−3)=119
(a+4)(b−3)=119
Now, the only 4 pairs of numbers that multiply to 119 are (1, 119), (7, 17), (17, 7), or (119, 1). This means that the only possibilities for a and b are (-3, 122), (3, 21), (13, 10), and (115, 4). The absolute difference for each of these numbers is 125, 18, 3, and 111 respectively. The smallest absolute difference out of all of these is 3
