(1)+(2)+(3)
2(1/a+1/b+1/c)=10
1/a+1/b+1/c=5 (4)
(4)-(2),1/a=-1
(4)-(3),1/b=2
(4)-(1),1/c=4
所以
a=-1/2
b=1/2
c=1/4