Equal charges q are placed at the four corners A,B,C,D of a square of length a. the magnitude of the force on the charge at B will be

$F_{net}=F_A+F_C+F_D$
$\mathrm{Since} F_A=F_C=\frac{k{q}^2}{a^2}\text{ and }{F}_D=\frac{k{q}^2}{(a\sqrt{2}{)}^2}$

And the forces F_A and F_C are acting mutually perpendicular to each other, 

The resultant of F_A and F_C will be along F_D

${\mathrm{F}}_{\mathrm{AC}}=\frac{\sqrt{2}{\mathrm{kq}}^2}{{\mathrm{a}}^2}$, So the net force will be along BD
${\mathrm{F}}_{\mathrm{net}}=\frac{\sqrt{2}{\mathrm{kq}}^2}{{\mathrm{a}}^2}+\frac{{\mathrm{kq}}^2}{2{\mathrm{a}}^2}=\frac{{\mathrm{kq}}^2}{{\mathrm{a}}^2}\left(\sqrt{2}+\frac{1}{2}\right)$
$=\frac{{\mathrm{q}}^2}{4{\mathrm{\pi \epsilon }}_0{\mathrm{a}}^2}\left(\frac{1+2\sqrt{2}}{2}\right)$