7 In an arithmetic progression the first term is 5 and the common difference is 3 . The \(n\)th term of the progression is denoted by \(u _ { n }\).
- Find the value of \(u _ { 20 }\).
- Show that \(\sum _ { n = 10 } ^ { 20 } u _ { n } = 517\).
- Find the value of \(N\) such that \(\sum _ { n = N } ^ { 2 N } u _ { n } = 2750\).