2) 9m2 + 6mn + n2 – 25 = (3m)2 + 2·3m·n + n2 – 25 =
= (3m + n)2 – 52 = (3m + n – 5)(3m + n + 5)
3) ab5 – b5 – ab3 + b3 = b5(a–1) – b3(a–1) =
= (b5 – b3)(a–1) = b3(b2 – 1)(a–1) = b3(b–1)(b+1)(a–1)
4) 1 – x2 + 10xy – 25y2 = 1 – (x2 – 2·x·5y + (5y)2) =
= 12 – (x – 5y)2 = (1 – x + 5y)(1 + x – 5y)