Ответ:
а) (x + y)(21 - 7y)
б) (a - 5 - 4p)(a - 5 + 4p)
в) (2b + c)(2b - c + 1)
Объяснение:
a) 21x - 7xy + 21y - 7y^2 = 21(x + y) - 7y(x + y) = (x + y)(21 - 7y)
б) a^2 - 10a + 25 - 4p^2 = (a - 5)^2 - 4p^2 = (a - 5 - 4p)(a - 5 + 4p)
в) 4b^2 + 2b + c - c^2 = 4b^2 - c^2 +2b +c = (2b - c)(2b + c) + 2b + c =
= (2b + c)(2b - c + 1)