\[m(l) = 2l^{2} + 3l;\ \ \ l = 25\ см.\]
\[1)\ l = 3\ см:\]
\[m^{'}(l) = 2 \bullet 2l + 3 = 4l + 3;\]
\[m^{'}(3) = 4 \bullet 3 + 3 = 15.\]
\[Ответ:\ \ 15\ г\text{/}см.\]
\[2)\ l = 25\ см:\]
\[m^{'}(l) = 2 \bullet 2l + 3 = 4l + 3;\]
\[m^{'}(25) = 4 \bullet 25 + 3 = 103.\]
\[Ответ:\ \ 103\ г\text{/}см.\]