Evalaute the following without using a calculator:
cos(13pi/6)

$$the\ cos\ function\ is\ periodic:\ \ \ the\ period=2 \pi\\\\cos(2 \pi + \alpha )=cos \alpha \\\\-\\\\cos\bigg{(} \frac{13 \pi }{6}\bigg{)}=cos\bigg{(} 2 \pi +\frac{ \pi }{6}\bigg{)}=cos\bigg{(} \frac{ \pi }{6}\bigg{)}= \frac{\sqrt{3} }{2}$$

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