3 An arithmetic series has fifth term 46 and twentieth term 181.
- Show that the common difference is 9 .
- Find the first term.
- Find the sum of the first 20 terms of the series.
- The \(n\)th term of the series is \(u _ { n }\).
Given that the sum of the first 50 terms of the series is 11525 , find the value of
$$\sum _ { n = 21 } ^ { 50 } u _ { n }$$