[1/tan(α/2)-tan(α/2)][1-2tanα(1/tan2α)] ={[1-tan(α/2)]/tan(α/2)}[1-2tanα(1-tanα)/2tanα] ={2[1-tan(α/2)]/2tan(α/2)}[1-(1-tanα)] =(2/tanα)tanα =2tanα