Three identical charges each of magnitude  3q0 are kept at vertices of an equilateral triangle. What should be charge at the centre to obtain system in equilibrium?

force on any charge due other two corner charges is 2 F cos 6020where F is equal to 3q02l2k  and it is repulsive  charge at the center should cancel this repulsive force by an attractive force is given by F1= k q3q0l32= 23q2l232k solving q=- q0 as the force has to be attractive