I guess you've made some mistake here...
I think what we have to prove is that
if so, take
for this you'll need the following equation,
eventually you'll get,
sin(theta+2alpha) + sin(theta) / sin(theta+2alpha) - sin(theta) = 1+n/1-n
using sin P + sin Q = 2sin[(P+Q)/2] cos[(P-Q)/2] and sin p - sin Q = 2cos[(P+Q)/2] sin[(P-Q)/2]
we get tan(theta + alpha)/tan( alpha) = 1+n/1-n and hence required result