a-b+c=0为式1,2a+2b-c=0为式2
由式1得a=b-c带入式2得b=3c/4,再带入式1得a=-c/4
3a-2b+5c=11c/4
5a-3b+2c=-3c/2
3a-2b+5c/5a-3b+2c=-11/6