\[\boxed{\mathbf{798}\mathbf{.}}\]
\[s(t) = \sqrt{t + 1} = (t + 1)^{\frac{1}{2}}\]
\[t = 3:\]
\[s^{'}(t) = \frac{1}{2} \bullet 1 \bullet (t + 1)^{\frac{1}{2} - 1} =\]
\[= \frac{1}{2} \bullet (t + 1)^{- \frac{1}{2}} = \frac{1}{2\sqrt{t + 1}}\]
\[s^{'}(3) = \frac{1}{2\sqrt{3 + 1}} = \frac{1}{2 \bullet \sqrt{4}} =\]
\[= \frac{1}{2 \bullet 2} = \frac{1}{4} = 0,25.\]
\[Ответ:\ \ 0,25.\]