7. A sequence is defined by
$$\begin{aligned}
u _ { 1 } & = 3
u _ { n + 1 } & = u _ { n } - 5 , \quad n \geqslant 1
\end{aligned}$$
Find the values of
- \(u _ { 2 } , u _ { 3 }\) and \(u _ { 4 }\)
- \(u _ { 100 }\)
- \(\sum _ { i = 1 } ^ { 100 } u _ { i }\)