Question

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

Answer 1

`cos0.2~=0.98`

Explanation:

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`