若 tana=5tanπ7\tan a = 5\tan \frac{\pi }{7}tana=5tan7π ，则 cos(a−5π14)sin(a−π7)=\frac{\cos \left( {a - \frac{5\pi }{14}}\right) }{\sin \left( {a - \frac{\pi }{7}}\right) } =sin(a−7π)cos(a−145π)=