Đáp án:
$\begin{array}{l}
a){S_{tp}} = 2.AB.BC + 2.AB.A'A + 2.BC.A'A\\
= 2.\left( {10.20 + 10.15 + 20.15} \right)\\
= 1300\left( {c{m^2}} \right)\\
b)A{C^2} = A{B^2} + B{C^2} = {10^2} + {20^2} = 500\\
\Rightarrow AC{'^2} = A{C^2} + A'{A^2}\\
= 500 + {15^2} = 725\\
\Rightarrow AC' = 26,93\left( {cm} \right)
\end{array}$