Răspuns:
5. d₁: x+y=1 <=> d₁: y= -x+1 (md₁= -1, n₁=1)
d₂: 3x-ay=2 <=> d₂: ay= 3x-2 <=> d₂: y= (3/a)*x - 2/a ( md₂= 3/a, n₂= -2/a)
d₁ ║d₂ => md₁= md₂ si n₁ ≠n₂
<=> [tex]\left \{ {{-1=\frac{3}{a} } \atop {1\neq \frac{-2}{a} }} \right. <=> \left \{ {{a=-3} \atop {a\neq -2}} \right.[/tex]
Deci, a=-3
