r1+r2+r3
a+b+c a+b+c a+b+c
2b b-a-c 2b
2c 2c c-a-b
c2-c1,c3-c1
a+b+c 0 0
2b -a-b-c 0
2c 0 -a-b-c
= (a+b+c)^3.