8. A geometric series has first term \(a\) and common ratio \(r\) where \(r > 1\).
The sum of the first \(n\) terms of the series is denoted by \(S _ { n }\).
Given that \(S _ { 4 } = 10 \times S _ { 2 }\),
- find the value of \(r\).
Given also that \(S _ { 3 } = 26\),
- find the value of \(a\),
- show that \(S _ { 6 } = 728\).