9. x+2y=8
x-2y=20 (inmultim a doua relatie cu 2)
adunam cele doua relatii
2x=28, x=28:2=14
14-2y=20, 2y=14-20, 2y=-6, y=-3
verificare
14+2x(-3)=14-6=8
10. (x+1)/x≥0, x≠0
x I -∞ -1 0 +∞
x+1 I---------------0++++++++++++++++++
x I----------------------------0++++++++++
(x+1)/x I++++++++0------------I+++++++++++
D=(-∞,-1]∪(0, +∞)