Răspuns:
x compus cu y=3xy-3x-3y+4
=3xy-3x-3y+3+1
=3(xy-x-y+1)+1
=3[x(y-1)-y+1]+1
=3[x(y-1)-1(y-1)]+1
=3(x-1)(y-1)+1
x compus cu (x+1)<=7
x compus cu (x+1)=3x(x-1)+1=3x^2-3x+1
3x^2-3x+1<=7
3x^2-3x-6<=0
3x^2-3x-6=0
Delta=9+72=81
X=(-b+radical din delta)/2a=12/6=2
X=(-b-radical din delta)/2a=-6/6=-1
=>x apartine {-1;2}
X compus cu y =7
X compus cu y =3(x-1)(y-1)+1
3(x-1)(y-1)+1=7
3(x-1)(y-1)=6 |:3
(X-1)(y-1)=2 => Caz1 x=0, y=0; caz2 x=2, y=2; caz3 x=3, y=1; c4 x=1; y=3, C5: x=-1, y=0, c6 x=0, y=-1
