2A+2B-7C=1
2A-7B+2C=2
-7A+2B+2C=3
(2A+2B-7C)-(2A-7B+2C)=1-2
9B-9C=-1
7(2A-7B+2C)+2(-7A+2B+2C)=7*2-2*3
-45B+18C=20
45(9B-9C)+9(45B+18C)=45*(-1)+9*20
-243C=135
C=-5/9
9B-9(-5/9)=-1
B=-2/3
2A+2(-2/3)-7(-5/9)=1
A=-7/9
所以A=-7/9,B=-2/3,C=-5/9