Why you are taking only inner plate charges of positively charged plate to calculate the electric field between the plates ? Is there no effect of that charges in electric field between the plates ? I am talking about red circled charges of the image attached.

I am talking about parallel plate capacitor. As we know that a positively charged plate creates electric field outside the plate by both the surface charges. So we get  total electric field at any point outside plate = sigma / 2K€ + sigma / 2K€ = sigma/K€. Also One surface of negativel charged plate creates electric field at the same point equals to sigma/ 2K€ . So i think total electric field between plates will be = 3sigma/2K€. If you say the plates are thin, then if we connect the uncharged plate to earth it will have neither negative nor positive charge on the grounded plate, so your concept fails to make parallel plate capacitor. So why we are not considering the outer surface charges of positively charged plate...? Please explain.