How do you evaluate $cos 870$ ?

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Answer 1 · 2016-10-05T14:51:10

$cos870^o=-sqrt3/2$

As trigonometric ratios get repeated over a cycle over $2pi$ or $360^o$ .

Hence $cos870^o=cos(360^o xx2+150^o)=cos150^o$

Further as $cos(180^o-A)=-cosA$

$cos870^o=cos150^o=cos(180^o-30^o)=-cos30^o=-sqrt3/2$

How do you evaluate cos 870cos 870?