the question isn't hard. Just do the double angles for cos(theta/2) which is 1-2sin^2(theta/2)
you should get 1-(1-2sin^2(theta/2)/1+1-2sin^2(theta/2)=2sin^2(theta/2)/2-2sin^2(theta/2). Clean it up factor the 2 out you get
sin^2(theta/2)/cos^2(theta/2)