11. The first three terms of an arithmetic series are \(60,4 p\) and \(2 p - 6\) respectively.
- Show that \(p = 9\)
- Find the value of the 20th term of this series.
- Prove that the sum of the first \(n\) terms of this series is given by the expression
$$12 n ( 6 - n )$$
\includegraphics[max width=\textwidth, alt={}, center]{e878227b-d625-4ef2-ac49-a9dc05c5321a-27_106_68_2615_1877}