1、(a-b)²-1
=(a-b+1)(a-b-1)
2、9a²-(2b+c)²
=(3a+2b+c)(3a-2b-c)
3、(2m-n)²-(m-2n)²
=(2m-n+m-2n)(2m-n)-m+2n)
=(3m-3n)(m+n)
=3(m-n)(m+n)