Answer
$4x^4$
Work Step by Step
$\sqrt {16x^8}=4x^4$
because,
$(4x^4)^2=16x^8$
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.