Physics---electrostatics2

E-field at A due to point charge B is Q/4πε0r2 in direction along BA

E-field at A due to point charge C is Q/4πε0r2 in direction along AC
Therefore the angle between the E-field vectors of charges B and C at A is 120° and hence, the resultant E-field will be in direction of 60° to each of the respective E-field since they have the same magnitude, i.e. in the direction parallel to BC.
Hence, the resultant E-field has a magnitude of:
2 × Q/4πε0r2 × cos 60° = Q/4πε0r2