全部代入,解三元一次方程就好了阿
a+b+c=4...(1)
a-b+c=0...(2)
(1)-(2)
2b=4,b=2
a+c=4-b=2.(3)
4a-2b+c=5
4a+c=5+2b=9.(4)
(4)-(3)
3a=7
a=7/3
c=-1/3