sin(2x) = 2sin(x)cos(x) ...... (1)
and
1 + cos(2x) =2cos2(x) ......(2) (why?)
=======================================
Reason:
----------
1 + cos(2x) = 1 + cos2(x) - sin2(x)
=> 1 + cos(2x) = cos2(x) + sin2(x) + cos2(x) - sin2(x)
.'. 1 + cos(2x) =2cos2(x)
========================================
(1) / (2) = sin(2x)/(1+ cos(2x))
= [2sin(x)cos(x)] / [2cos2(x)]
= tan(x)
Q.E.D