2) 9m^2 + 6mn + n^2 - 25 = (3m)^2 + 2*3m*n + n^2 - 25 =
= (3m + n)^2 - 5^2 = (3m + n - 5)(3m + n + 5)
3) ab^5 - b^5 - ab^3 + b^3 = b^5(a-1) - b^3(a-1) =
= (b^5 - b^3)(a-1) = b^3(b^2 - 1)(a-1) = b^3(b-1)(b+1)(a-1)
4) 1 - x^2 + 10xy - 25y^2 = 1 - (x^2 - 2*x*5y + (5y)^2) =
= 1^2 - (x - 5y)^2 = (1 - x + 5y)(1 + x - 5y)