(a)[4]
Show that the common difference equals $-\frac{1}{\cos^4 \theta}$.
(b)[3]
Find the exact value of the 13th term for $\theta = \frac{1}{6}\pi$.
Mathematics 9709 · AS & A Level · Series
Series — practice question
The first two terms in an arithmetic progression are $\frac{1}{\cos^2 \theta}$ and $-\frac{\tan^2 \theta}{\cos^2 \theta}$, in that order, where $0 < \theta < \frac{1}{2}\pi$.