Đáp án:
$1) a=-91$
$2) a=-20$
$3) a= \frac{-14}{3}$
$4) a= 11$
$5) a=\frac{52}{3}$
Giải thích các bước giải:
$1) a+b-c=18\\ \Rightarrow a=18-b+c\\ =18-100+(-9)=-91\\
2) 2a-3b+c=0\\ \Rightarrow 2a=3b-c\\ =3.(-12)-4=-36-4\\ =-40\\ \Rightarrow a=\frac{-40}{2}=-20\\
3) 3a-b-2c=2\\ \Rightarrow 3a=2+b+2c\\ =2+6+2.(-11)\\=8-22\\ =-14\\ \Rightarrow a=\frac{-14}{3}\\
4) 12-a+b+5c=-1\\ \Rightarrow a=12+b+5c+1\\ =12+(-27)+5.5+1\\=12-27+25-1\\ =12-2+1=11\\
5)1-2b+c-3a=-9 \\ \Rightarrow 3a=1-2b+c+9\\=1-2.(-23)+(-4)+9\\=1+46-4+9\\ =47+5=52\\ \Rightarrow a=\frac{52}{3}$