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 sin780°  , cos 13П/6

[tex]sin780^0=sin(720^0+60^0)=sin(4 \pi +60^0)=sin60^0= \frac{ \sqrt{3} }{2} [/tex]

[tex]cos \frac{13 \pi }{6} =cos( \frac{12 \pi }{6}+ \frac{ \pi }{6})=cos(2 \pi + \frac{ \pi }{6})=cos \frac{ \pi }{6}= \frac{ \sqrt{3} }{2} [/tex]