(1/cost+tgt)(1/cost-tgt)
(1/cost)+tgt=(1/cost)+(sint/cost)=(1+sint)/cost (1/cost)-tgt=(1/cost)-(sint/cost)=(1-sint)/cost ((1/cost)+tgt)*((1/cost)-tgt)= =(1+sint)*(1-sint)/cos^2t=(1-sin^2t)/cos^2t= =cos^2t/cos^2t=1