Chú ý rằng: \(sin{45^0} = {\rm{ }}cos{45^0},{\rm{ }}sin{40^0} = {\rm{ }}cos{50^0},\)\({\rm{ }}sin{50^0} = {\rm{ }}cos{40^0}\)
Ta được:
\({{\cos {{50}^0} - \cos {{45}^0} + \cos {{50}^0}} \over {\cos {{40}^0} - \cos {{45}^0} + \cos {{50}^0}}}- {{6.3({{\sqrt 3 } \over 3} + \tan {{15}^0})} \over {3(1 - {{\sqrt 3 } \over 3}\tan {{15}^0})}}\)
\(\eqalign{
& = 1 - 6({{\tan {{30}^0} + \tan {{15}^0}} \over {1 - \tan {{30}^0}.\tan {{15}^0}}}) \cr
& = 1 - 6\tan {45^0} = - 5 \cr} \)
