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## Homework Statement

[tex] \cos (2 \theta)=- \cos ( \theta)[/tex]

## Homework Equations

## The Attempt at a Solution

In general [tex] \theta = 2n \pi +/- \alpha \mbox{ where } \cos \theta = \cos \alpha [/tex]

Because [tex] \cos(2 \theta) = - \cos( \theta) \mbox{ then, } 2 \theta = 2n \pi -/+ \theta[/tex]

taking [tex] 2 \theta = 2n \pi - \theta \mbox{ that implies } 3 \theta = 2n \pi \mbox{ therefore } \theta = \frac{2}{3}n \pi [/tex]. The book says that theta must lie between 0 and pi/2. So I must have gone wrong. Can anyone put me right on this equation. Thanks for the help.