\$2n\$ point charges on a circle at \$O\$, what's total electric field intensity \$\overline{E}\$ at \$\pm 1\mathrm{m}\$?
I see my error.  The intensity in the direction from a point charge on the circle to \$z = \pm 1 m\$ is \$4.5 V/m\$.  Divide by \$\sqrt{2}\$ or multiply by \$\cos \pi/4\$ to the the component of intensity in the \$z\$ direction.  \$4.5 / \sqrt{2} \approx 3.18\$.