D =
c1-2c2+c3
(a-b)^2 ab b^2
0 a+b 2b
0 1 1
c2-c3
= (a-b)^2 *
(a-b)^2 ab-b^2 b^2
0 a-b 2b
0 0 1
= (a-b)^3