1、4a-4a²-1
=-[(2a²)-4a+1]
=-(2a-1)²
2、25a²-4(b+c)²
=[5a+2(b+c)][5a-2(b+c)]
=(5a+2b+2c)(5a-2b-2c)
3、(x+y)²+6(x+y)+9
=(x+y+3)²