\[\boxed{\text{792.}\text{\ }\text{еуроки}\text{-}\text{ответы}\text{\ }\text{на}\text{\ }\text{пятёрку}}\]
\[a + b = 40\]
\[a = 40 - b\]
\[a \cdot b = b(40 - b) =\]
\[= - b^{2} + 40b =\]
\[= - \left( b^{2} - 2 \cdot 20b + 400 - 400 \right) =\]
\[= - \left( (b - 20)^{2} - 400 \right) =\]
\[= 400 - (b - 20)^{2} \leq 400.\]
\[Наибольшее\ значение\ 400\ \]
\[при\ \ (b - 20)^{2} = 0 \Longrightarrow\]
\[\Longrightarrow b = 20 \Longrightarrow a = 40 - b =\]
\[= 40 - 20 = 20.\]
\[Ответ:при\ a = 20;b = 20.\]