Calculation of the electrostatic forces

Two equal point charges A and B are R distance apart. A third point charge placed on the perpendicular bisector at a distance D from the centre will experience maximum electrostatic force when D equals to what?

[WingsTK]

I was looking forever to see how to find the maximum and this video helped a lot!

[greyy_matter]

Dude how did u get the differential..??

[rampant]

Thanks for the video, 1 am here and this is helping a lot :)

[whysrinu]

Can you please tell us how to differentiate it ?

[hamzasaim]

First derivative also gives minimum force... So how will you differ...???