Explanation:
1/(seca-tana) - 1/cosa=1/cosa -1/(seca+tana)
There are many ways to solve this, let's take a look at a simple one
1/(seca-tana) + 1/(seca+tana) =1/cosa + 1/cosa
((seca-tana)+(seca+tana))/(sec^2a-tan^2a)=2/cosa
(seca-tana+seca+tana)/1=2*1/cosa
2seca=2seca