$\vec{a}\times \vec{b}=\begin{array}{ccc}\hfill \vec{x}\hfill & \hfill \vec{y}\hfill & \hfill \vec{z}\hfill \\ \hfill 1\hfill & \hfill 2\hfill & \hfill 8\hfill & \hfill 1\hfill & \hfill 2\hfill \\ \hfill 4\hfill & \hfill 3\hfill & \hfill 5\hfill & \hfill 4\hfill & \hfill 3\hfill \\ \hfill \hfill \end{array}=\left(\begin{array}{c}\hfill 10-24\hfill \\ \hfill 32-5\hfill \\ \hfill 3-8\hfill \\ \hfill \hfill \end{array}\right)$
$=\left(\begin{array}{c}\hfill -14\hfill \\ \hfill 27\hfill \\ \hfill -5\hfill \\ \hfill \hfill \end{array}\right)$
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$A=\sqrt{14^2+27^2+5^2}=\sqrt{950}\approx 30,8$.