7 The first and second terms of an arithmetic progression are \(\frac { 1 } { \cos ^ { 2 } \theta }\) and \(- \frac { \tan ^ { 2 } \theta } { \cos ^ { 2 } \theta }\), respectively, where \(0 < \theta < \frac { 1 } { 2 } \pi\).
- Show that the common difference is \(- \frac { 1 } { \cos ^ { 4 } \theta }\).
- Find the exact value of the 13th term when \(\theta = \frac { 1 } { 6 } \pi\).