If in the three phase connection phase to phase voltage is 400v what will be the phase neutral voltage?

The formula to use is, phase voltage /1.73 = phase to neutral (ground) voltage.

There is no such thing as a 'phase to phase', or 'phase to neutral' voltage. The correct terms are 'line to line' and 'line to neutral'. So the above answer should read: line voltage/1.73= line to neutral voltage = phase voltage.