Consider the charges q, q and -q placed at the vertices of an equilateral triangle of each side l. The sum of forces acting on each charge is

Explanation

Force on -q is
`vec(F)= vec(F)_(1)+ vec(F)_(2) cos 30^(@) (F_(1)=F_(2))`
`= ( sqrt( 3)q^(2))/( 4pi epsilon_(0)L^(2))`
Force on each of `+q` is
`vec(F)= vec(F)_(1) + vec(F)_(2)`
`2 F_(1) cos 60^(@) ( F_(1) = F_(2))`
`= 2 xx ( q^(2))/( 4pi epsilon_(0)L^(2)) xx(1)/(2)`
`= (q^(2))/( 4pi epsilon_(0)L^(2))`