1+tg^2a+1/sin^2a=1/cos^2a+1/sin^2a=1/sin^2acos^2a
(1+tg^2 a+1\sin a)*sin^2 a cos^2 a=(1/sin^2acos^2a)*sin^2 a cos^2 a=1
1+tg^2a+1/sin^2a=1/cos^2a+1/sin^2a=1/sin^2acos^2a
(1+tg^2 a+1\sin a)*sin^2 a cos^2 a=(1/sin^2acos^2a)*sin^2 a cos^2 a=1