Решение.\[ \begin{align}
& \vec{F}=4\cdot t\cdot \vec{i}+3\cdot \cos (\pi \cdot t)\cdot \vec{j}, \\
& F=\sqrt{{{(4\cdot t)}^{2}}+{{(3\cdot \cos (\pi \cdot t))}^{2}}}\ \ \ (1). \\
& F=\sqrt{{{(4\cdot 1)}^{2}}+{{(3\cdot \cos \pi )}^{2}}}=\sqrt{{{(4)}^{2}}+{{(3\cdot (-1))}^{2}}}=5,0\ H. \\
\end{align} \]