25x²—16y²
=(5x)^2-(4y)^2
=(5x+4y)(5x-4y)
(a—b)(x—y）—（b—a）（x+y)
=(a—b)(x—y）+（a-b）（x+y)
=(a-b)(x-y+x+y)
=2x(a-b)
a²—4ab+4b²
=(a-2b)^2
=(a-2b)(a-2b)
4+12（x—y）+9(X—y）²
=[(3(x-y)+2]^2
=[(3(x-y)+2][(3(x-y)+2]