(x+y)(a-b-c)+(x-y)(b+c-a)=(x+y)(a-b-c)-(x-y)(a-b-c)=(a-b-c)[(x+y)-(x-y)]=(a-b-c)·2y=2(a-b-c)y
(x-y)(a+b+c)
1+1=2sytrsss
