Answer:
Step-by-step explanation:
[tex]cos(\frac{13\pi }{12} )\\=cos(\pi +\frac{\pi }{12} )\\=-cos(\frac{\pi }{12} )\\=-cos(\frac{\pi }{3} -\frac{\pi }{4} )\\=-[cos \frac{\pi }{3} cos\frac{\pi }{4} +sin\frac{\pi }{3} sin\frac{\pi }{4} ]\\=-[\frac{1}{2} *\frac{1}{\sqrt{2} } +\frac{\sqrt{3} }{2} *\frac{1}{\sqrt{2} } ]\\=-\frac{1+\sqrt{3} }{2\sqrt{2} }[/tex]
