## Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

$0.6$
As $\cos(\theta+2\pi)=\cos(\theta+4\pi)=\cos\theta$, we have $\cos\theta+\cos(\theta+2\pi)+\cos(\theta+4\pi)=\cos{\theta}+\cos{\theta}+\cos{\theta}=3\cos\theta=3(0.2)=0.6$