\[2 \cdot (a + b) = 26\]
\[(a + b) = 13\]
\[a = 13 - b.\]
\[S = a \cdot b = b(13 - b) = 36\]
\[13b - b^{2} = 36\]
\[b^{2} - 13b + 36 = 0\]
\[b_{1} + b_{2} = 13;\ \ b_{1} \cdot b_{2} = 36\]
\[b_{1} = 4;\ \ \ b_{2} = 9.\]
\[a_{1} = 13 - b = 13 - 4 = 9;\]
\[a_{2} = 13 - b = 13 - 9 = 4.\]
\[Ответ:стороны\ прямоугольника\ \]
\[равны\ 4\ см\ и\ 9\ см.\]