a+b+c=0 4a+2b+c=3 25a+5b+c=60

a+b+c=0 4a+2b+c=3 25a+5b+c=60
a+b+c=0 4a+2b+c=3 25a+5b+c=60

a+b+c=0 4a+2b+c=3 25a+5b+c=60
a+b+c=0 (1)
4a+2b+c=3 (2)
25a+5b+c=60 (3)
(2) - (1)
3a + b = 3
b = 3-3a
c = -a -3+3a = 2a -3
代入 (3)
a = 4
b = -9
c = 5