785. Разложите на множители: a) $$(a – 3b)(a + 2b) + 5a(a + 2b)$$; б) $$(x + 8y)(2x - 5b) – 8y(2x – 5b)$$; в) $$7a^2(a – x) + (6a^2 – ax)(x – a)$$; г) $$11b^2(3b – y) – (6y - 3b^2)(y - 3b)$$.

**a) $$(a – 3b)(a + 2b) + 5a(a + 2b)$$** Выносим $$(a + 2b)$$ за скобки: $$(a + 2b)(a - 3b + 5a) = (a + 2b)(6a - 3b) = 3(a + 2b)(2a - b)$$. **б) $$(x + 8y)(2x - 5b) – 8y(2x – 5b)$$** Выносим $$(2x - 5b)$$ за скобки: $$(2x - 5b)(x + 8y - 8y) = (2x - 5b)(x) = x(2x - 5b)$$. **в) $$7a^2(a – x) + (6a^2 – ax)(x – a)$$** $$7a^2(a - x) - (6a^2 - ax)(a - x) = (a - x)(7a^2 - 6a^2 + ax) = (a - x)(a^2 + ax) = a(a - x)(a + x)$$. **г) $$11b^2(3b – y) – (6y - 3b^2)(y - 3b)$$** $$11b^2(3b - y) + (6y - 3b^2)(3b - y) = (3b - y)(11b^2 + 6y - 3b^2) = (3b - y)(8b^2 + 6y) = 2(3b - y)(4b^2 + 3y)$$.