Consider the charges q, q and -q placed at vertices of an equilateral triangle as shown it figure. What is the force on each charge ?

Explanation

The forces acting on charge q at a due to charges q at B and -q at c are `F_(12)` along BA and `F_(13)` along AC respectively q at a is given by
the force of attaraction or repulson for each pari of charges has the same magnitude `F=(q^(2))/(4piepsilon_(0)l^(2))`
The total force `f_(2)` on charge q at b is thus `f_(2)` =f `vecr_(2)` where `vecr_(2)` is a unit vector along AC
`F_(1)+F_(2)+F_(3)=0`
The result is not at all surprising it follows straight from the fact that colulomb law is consistent with newton thrid law the proof is left to you as an exercise