9. The third term of an arithmetic series is \(5 \frac { 1 } { 2 }\).
The sum of the first four terms of the series is \(22 \frac { 3 } { 4 }\).
- Show that the first term of the series is \(6 \frac { 1 } { 4 }\) and find the common difference.
- Find the number of positive terms in the series.
- Hence, find the greatest value of the sum of the first \(n\) terms of the series.