find tan^-1(xy/(1+x^2+y^2)^1/2)?

Question

find the sum of the series1. 1-1cos2a/2+(1*3 cos4a)/2*4-...........
2. csina+c^3sin3a/3+c^5sin5a/5+.............

Answer 1

try reducing the multiple angle sines and cosines to sines and cosines raised to a power. It might pay off and these might reduce to known forms of series. If not you need to attack it rigorously with analytic methods.
And the first part makes no sense.