Câu hỏi 8
Với mọi góc $0 \lt \theta \lt \pi / 2$, chứng minh rằng
$$
0 \lt \sin \theta + \cos \theta + \tan \theta + \cot \theta - \sec \theta - \csc \theta \lt 1
$$
$$
0 \lt \sin \theta + \cos \theta + \tan \theta + \cot \theta - \sec \theta - \csc \theta \lt 1
$$