12.15. На 1,2,3. Пожалуйста.

1) a(2a+b)(a+b) - 4a(a+b)^2 = a*(a+b)*(2a+b-4(a+b)) = a*(a+b)*(2a+b-4a-4b) =

= a*(a+b)*(-2a-3b);

2) 3m^2(m-8) + 6m(m-8)^2 = 3m*(m-8)*(m+2(m-8)) = 3m*(m-8)*(m+2m-16) =

= 3m * (m-8)  * (3m-16);

3) (2a+3)(a+5) + (a-1)(a+5) = (a+5) * (2a+3+a-1) = (a+5)*(3a+2).