x-25=(x+5)(x-5)a-ab=a(a-b) = a(a+b)(a-b) ① m-9n = (m+3n)(m-3n) ② ab-4ab = ab(b-4) ③ (5x-3)-16x = (5x+4√x-3)(5x-4√x-3) ④ 16x^4-1 = (4x+1)(4x-1) = (4x+1)(2x+1)(2x-1)