c1+c2+c3
2(x+y) y x+y
2(x+y) x+y x
2(x+y) x y
r3-r2,r2-r1
2(x+y) y x+y
0 x -y
0 -y y-x
--此时用对角线法则得
= 2(x+y)[x(y-x)-y^2]
= -2(x^3+y^3).