1、(m²+n²)²-4m²n²
= (m²+n²-2mn) (m²+n²+2mn)
= (m-n)² (m+n)²
2、 x²+2x+1-y²
=(x+1)²-y²
=(x+1-y)(x+1+y)
3、 25（a-b）²+10（a-b）+1
=[5(a-b)+1]²
=(5a-5b+1)²