（c-a）的平方-4乘（b-c)乘（a-b）用因式分解怎么做?

设b-c=x,a-b=y,c-a=-(x+y )
原式=(x+y)^2-4xy
=x^2+y^2+2xy-4xy
=x^2-2xy+y^2
=(x-y)^2
=[(b-c)-(a-b)]^2
=(2b-c-a)^2