a=r.cosx, find tan(X/2)

tan(2x) = 2tanx / 1 - (tan^2 (x))

using this tanx = 2 tan (x/2) / (1- tan^2 x/2)

assume tan(x/2) = t

you will get tanx = 2t/ 1-(t^2)

find secx next then find cosx, then substitute cosx value, you will get answer