Answer
$$ y'= - \frac{ 1}{ x\sqrt{1-(\ln x)^2}}.$$
Work Step by Step
Since $ y=\ arcos( \ln x)=\cos^{-1}( \ln x) $, then
$$ y'=- \frac{1}{ \sqrt{1-(\ln x)^2}}(\ln x)'\\
=- \frac{ 1/x}{ \sqrt{1-(\ln x)^2}}=- \frac{ 1}{ x\sqrt{1-(\ln x)^2}}.$$
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