①
x^4(a-2b)+x^2y^2(2b-a)
=x^4(a-2b)-x^2y^2(a-2b)
=x^2(a-2b)(x^2-y^2)
=x^2(a-2b)(x+y)(x-y)
②
3a（x^2+4）^2-48ax^2
=3a[(x^2+4)^2-4^2x^2]
=3a(x^2-4x+4)(x^2+4x+4)
=3a(x-2)^2(x+2)^2
③
16（a-b）^2-25*（a+b）^2
=4^2(a-b)^2-5^2(a+b)^2
=(4a-4b+5a+5b)(4a-4b-5a-5b)
=-(9a+b)(a+9b)