1) (a+2b)²-(3c+4d)²=(a+2b-(3c+4d))(a+2b+3c+4d)=(a+2b-3c-4d)(a+2b+3c+4d).
2) (m-2n)²-(2p-3q)²=(m-2n-(2p-3q))(m-2n+2p-3q)=(m-2n-2p+3q)(m-2n+2p-3q).
3) 9(m+n)²-(m-n)²=(3(m+n)-(m-n))(3(m+n)+m-n)=(3m+3n-m+n)(3m+3n+m-n)=(2m+4n)(4m+2n)=2(m+2n)·2(2m+n)=4(m+2n)(2m+n).
4) 16(a+b)²-9(x+y)²=(4(a+b)-3(x-y))(4(a+b)+3(x+y))=(4a+4b-3x-3y)(4a+4b+3x+3y).