Ответ:

1) 81-(c-d)²=(9-(c-d))×(9+(c-d))=(9-c+d)(9+c-d)

2) m²-(m+n)²=(m-m-n)×(m+m+n)=-n(2m+n)

3) m⁴-n⁴=(m²-n²)×(m²+n²)=(m-n)×(m+n)×(m²+n²)

4) 4m²-(m+n)²=(2m-(m+n))×(2m(m+n))=

(2m-m-n)×(2m+m+n)=(m-n)(3m+n)

5) (2m-3n)²-(4n-5m)²=(2m-3n-(4n-5m))×(2m-3n+(4n-5m))=

(7m-7n)×(-3m+n)=7(m-n)+(-3m+n)