# How do you evaluate cos^-1(cos((3pi)/10))?

${\cos}^{-} 1 \left(\cos \left(\frac{3 \pi}{10}\right)\right) = \frac{3 \pi}{10}$
Use a fundamental property of inverses: ${f}^{-} 1 \left(f \left(x\right)\right) = x$.