opposite sides and angles in a || are equal.
sum of adjcent sides is equal to 180
the bisectors of these angles form a triangle. whose two angles are A/2 and B/2 (As it is been bisected), or A/2 and (180 - A)/2 = (90 - A/2)
A + B + C = 180
for the above case,
we have A/2 + 90 - A/2 + C = 180.
C=180 - 90
C = 90
Hence, the two bisectors intersect at right angles.