tan2 θ(sec θ−1)2=
1+sin θ1−sin θ
1
tan θ
1+cos θ1−cos θ
tan2 θ(sec θ−1)2=sec2 θ−1(sec θ−1)2
=(sec θ−1)(sec θ+1)(sec θ−1)(sec θ−1)
=1cos θ+11cos θ−1
=1+cos θcos θ1−cos θcos θ
=1+cos θ1−cos θ