## Precalculus (10th Edition)

$\sqrt{21}$
The distance $d$ from $P_1(x_1,y_1,z_1)$ to $P_2(x_2,y_2,z_2)$ is $d=\sqrt{(x_2-x_1)^2+(y_2-y_1)^2+(z_2-z_1)^2}$. Hence, here $d=\sqrt{(4-0)^2+(1-0)^2+(2-0)^2}\\ d=\sqrt{4^2+1^2+2^2}\\ d=\sqrt{16+1+4}\\ d=\sqrt{21}$.