How do you evaluate $Cos 0.2 + cos (0.2+pi)+ cos (0.2+4 pi)$ ?

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Answer 1 · 2016-11-24T07:45:38

$cos0.2~=0.98$

Since

$cos(pi+alpha)=-cosalpha$

and $cos(4pi+alpha)=cos(2pi+alpha)=cos(alpha)$ ,

the given expression is equivalent to the following:

$cos0.2cancel(-cos0.2)cancel(+cos0.2)=cos0.2~=0.98$

How do you evaluate Cos 0.2 + cos (0.2+pi)+ cos (0.2+4 pi)Cos 0.2 + cos (0.2+pi)+ cos (0.2+4 pi)?