6. Let a be a real number. Consider the series 00 a Z on, where on = cos('mr) 2n + 1. n=0 (a) Is it possible to nd an a > 0 such that the above series is both absolutely convergent and conditionally convergent? Briey explain your reasoning. Answers without reasoning will be given 0. (b) Find all a > 0 such that the series diverges. (c) Find all a > 0 such that the series converges absolutely. (d) Find all a > 0 such that the series converges conditionally