Question

Daca 2a+b=4 si a+b=8,calculati 3a+2b-7

Answer 1

$\left \{ {{2a+b=4} \atop {a+b=8}} \right.$⇒$\left \{ {{2a+b=4} \atop {2a+2b=16}} \right.$
Le scadem si obtinem:
-b=-12⇒b=12
2a+b=4
2a+12=4
2a=-8⇒a=-4
3a+2b-7=-12+24-7=5

Answer 2

$2a+b=4 \\ b=4-2a \\ a+b=8 \Rightarrow a+4-2a=8 \Rightarrow a-2a=8-4 \Rightarrow -a=4 \Rightarrow a=-4 \\ b=4-2a \Rightarrow b=4-2 \cdot (-4) \Rightarrow b=4+8=12 \\ 3a+2b-7=3 \cdot (-4)+2 \cdot 12-7=-12+24-7=12-7=5 \\ \boxed{3a+2b-7=5}$