(x + 4)(y - 1) = xy
xy - x + 4y - 4 = xy
-x + 4y = 4 .(1)
(x - 2)(y + 1) = xy
xy + x - 2y - 2 = xy
x - 2y = 2 .(2)
(1) + (2)得
2y = 6
y = 3 .代入(2)得
x = 8