1. Tính A=$sin^{2}50^{o}+sin^{2}70^{o}-cos50^{o}cos70^{o}$
2.Tính A= $4cos15^{o}cos21^{o}cos24^{o}-cos12^{o}-cos18^{o}$
3.Tính A= $\frac{1}{sin18^{o}}-\frac{1}{sin54^{o}}$
4. Tính A= $tan9^{o}-tan27^{o}-tan63^{o}+tan81^{o}$
5. CMR $\frac{sin(a-b)}{cosacosb}+\frac{sin(b-c)}{cosbcosc}+\frac{sin(c-a)}{cosccosa}=0$
6. CMR $\sum \frac{sina}{sin(a-b)sin(a-c)}=0$
7. CMR $\sum_{k=1}^{10}sin\frac{k\pi }{11}=cot\frac{\pi }{22}$
8. Tính A= $\frac{sin35^{o}sin65^{o}sin80^{o}}{sin20^{o}+sin50^{o}+sin70^{o}}$