(1)64m^2 n^2-(m^2+16n^2)^2
=(8mn-m²-16n²)(8mn+m²+16n²)
=-(4n-m)²(4n+m)²
(2)a^4-2a^2 b^2+b^4
=(a²-b²)²
=(a-b)²(a+b)²