Find series expansion of 1/cosx Find the series expansion of 1/cosx from basic series expansions.
I tried to find 1/cosx from the expansion of cosx but was unsure how to continue. When I found 1/cosx from the basic formula for finding series expansions I didn't get the same answer.

$\displaystyle\dfrac1{\cos x}=\sum_{n=0}^\infty a_{n}\dfrac{x^{2n}}{(2n)!},~$ where $a_n$ form this sequence. See Euler number or zigzag number for 
more details.
