\(\eqalign{
& \cos {{ - 11\pi } \over 4} = \cos ( - 2\pi - {{3\pi } \over 4}) = \cos ( - {{3\pi } \over 4}) = \cos ({{3\pi } \over 4}) = {{ - \sqrt 2 } \over 2} \cr
& \tan {{31\pi } \over 6} = \tan (4\pi + \pi + {\pi \over 6}) = \tan {\pi \over 6} = {{\sqrt 3 } \over 3} \cr
& \sin ( - {1380^0}) = sin( - {4.360^0} + {60^0}) = \sin {60^0} = {{\sqrt 3 } \over 2} \cr} \)
