Answer
$2x\sqrt5$
Work Step by Step
$\sqrt 10x\times\sqrt2x=\sqrt(10x\times2x)=\sqrt (20x^{2})=\sqrt (4x^{2}\times5)=\sqrt (4x^{2})\times \sqrt5=2x\sqrt5$
We know that $\sqrt(4x^{2})=2x$, because $(2x)^{2}=2^{2}x^{2}=4x^{2}$.
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