911. Представьте в виде дроби: a) $$\frac{ab^2-16a}{5b^3} \cdot \frac{20b^5}{a^2b+4a^2}$$; б) $$\frac{7xy}{x^2-4xy+4y^2}:\frac{3x-6y}{14y^2}$$.

a) $$\frac{ab^2-16a}{5b^3} \cdot \frac{20b^5}{a^2b+4a^2}=\frac{a(b^2-16)}{5b^3} \cdot \frac{20b^5}{a^2(b+4)}=\frac{a(b-4)(b+4)}{5b^3} \cdot \frac{20b^5}{a^2(b+4)}=\frac{20ab^5(b-4)(b+4)}{5a^2b^3(b+4)}=\frac{4b^2(b-4)}{a}$$
б) $$\frac{7xy}{x^2-4xy+4y^2}:\frac{3x-6y}{14y^2}=\frac{7xy}{(x-2y)^2}:\frac{3(x-2y)}{14y^2}=\frac{7xy}{(x-2y)^2} \cdot \frac{14y^2}{3(x-2y)}=\frac{98xy^3}{3(x-2y)^3}$$