Answer
$\pi$
Work Step by Step
Use the properties of an inverse function, $cos^{-1}(tan(\frac{3\pi}{4}))=cos^{-1}(-tan(\frac{\pi}{4}))=cos^{-1}(-1)=\pi$
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.