a) 7(x - 1) + 3y = 2xy
<=> 2xy - 7x - 3y + 7 = 0
<=> 4xy - 14x - 6y + 14 = 0
<=> 2x(2y - 7) - 3(2y - 7) = 7
<=> (2x - 3)(2y - 7) = 7
@ 2x - 3 = - 1; 2y - 7 = - 7 <=> x = 1; y = 0
@ 2x - 3 = - 7; 2y - 7 = - 1 <=> x = - 2; y = 3
@ 2x - 3 = 1; 2y - 7 = 7 <=> x = 2; y = 7
@ 2x - 3 = 7; 2y - 7 = 1 <=> x = 5; y = 4
KL : Pt có 4 nghiệm nguyên (x; y) = (1; 0); (- 2; 3); (2; 7); (5; 4)