I4.4 If $u=\sqrt{t(t+1)(t+2)(t+3)+1}$, find $u$.
$\begin{aligned} u & =\sqrt{5 \times 6 \times 7 \times 8+1}=\sqrt{40 \times 42+1} \\ & =\sqrt{(41-1) \times(41+1)+1}=\sqrt{41^{2}-1+1} \\ u & =41\end{aligned}$