din proportia x/2=1/2 obtinem y=2·x (*)
a) daca x+y=6, inlocuim pe y din (*) si obtinem:
x+2·x=6 <=> 3·x=6 =>x=6:2 => x=2,
revenim la (*) si y=2·2 => y=4.
b) y-x=6 <=> 2·x-x=6 <=> x=6
=> y=2·6, y=12.
c)3x+2y=42 <=> 3x+2·2x=42 <=> 3x+4x=42 <=> 7x=42 => x=42:7 => x=6.
=> y=2·6, y=12.
d) 2y-3x=6 <=> 2·2x-3x=6 <=> 4x-3x=6 <=> x=6 => y=2·6 => y=12.