Answer
$(9u^2+1)(3u+1)(3u-1)$
Work Step by Step
$81u^4-1=(9u^2)^2-1^2=(9u^2+1)(9u^2-1)=(9u^2+1)[(3u)^2-1^2]=(9u^2+1)(3u+1)(3u-1)$
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