Answer
$\sqrt {21}$
Work Step by Step
In this case $P_{1}$ is at origin. So the distance formula reduces to d= $\sqrt {x^{2}+y^{2}+z^{2}}$.
Substituting $x= 4, y= 1,$ and $z=2$, we obtain
$d= \sqrt {4^{2}+1^{2}+2^{2}}= \sqrt {21}$
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