sin2theta-tan2theta
=(2tantheta)/(1+tan^2theta)-(2tantheta)/(1-tan^2theta)
=(2tantheta(1-tan^2theta-1-tan^2theta))/((1+tan^2theta)
(1-tan^2theta))
=(-2tantheta(2tan^2theta))/((1+tan^2theta)(1-tan^2theta))
=-(4tan^3theta)/((1+tan^2theta)(1-tan^2theta))
=(4tan^3theta)/(tan^4theta-1)