(a^2+b^2)^2-4a^2b^2
=(a^2+b^2+2ab)(a^2+b^2-2ab)
=(a+b)^2*(a-b)^2
这题打错了个数字吧
m^2n^2/9+2mn^3/3+n^4
=1/9*n^2*(m^2+6mn+9n^2)
=1/9*n^2*(m+3n^2)^2
25a^2-10a+1
=(5a-1)^2
(x^2+1)^2-4x^2
=(x^2+1+2x)(x^2+1-2x)
=(x+1)^2*(x-1)^2