Principle of superposition

Find the force on any one of the charges if three charges each of 5C is placed on the vertices of an equilateral triangle of side 3cm( figure)

Let us consider the 5C charge at B

F₁ be the force acting on 5C charge at B by A and F₂ be the force acting on charge B by charge at C

since it is an equilateral triangle  and all charges the same F₁=F₂=F

from the figure these two forces F is inclined with each other at an angle of 60⁰ (since equilateral triangle)





•^•• by the parallelogram law of forces resultant 

R=SQRT(F²+F²+2FFcos60)=√3F

but F=(1/4πε₀)(Q×q)/r^-2

ie F=(1/4πε₀)(5×5)/9×10^-4

F=9×10⁹×25×10⁴/9=25×10¹³ N

The resultant force  R,on charge at B=25√3×10¹³ N ( the force on each charge on the other vertices also will be the same)