If the bisector of angles A and B of a quadrilateral ABCD intersect each other at the point P. The point P is equidistant from AD and BC.
From P, perpendiculars PL on AD, PM on AB and PN on BC respectively are drawn.
P lies on the bisector of angle A, hence, PL = PM -------------------- (i)
Again, P lies on bisector of angle B, PM = PN ------------------------ (ii)
From (i) and (ii) we get, PL = PN i.e. P is equidistant from AD and BC. [Proved.]