Tính giá trị

$$\begin{gathered} \mathrm{Tính}: \\ \mathrm{a})\mathrm{A}={\left(\frac{3}{5}\right)}^2.5^2-{\left(2\frac{1}{4}\right)}^3:{\left(\frac{3}{4}\right)}^3+\frac{1}{2}. \\ \mathrm{b})\mathrm{B}={\left[\frac{4}{11}.{\left(\frac{1}{25}\right)}^0+\frac{7}{22}.2\right]}^{2021}-{\left(\frac{1}{2^2}-\frac{8^3}{4^4}\right)}^{2020} \end{gathered}$$