Answer:
Step-by-step explanation:
Given:    A line m is perpendicular to the angle bisector of ∠A. We call this Â
         intersecting point as D. Hence, in figure ∠ADM=∠ADN =90°.
         AD is angle bisector of ∠A. Hence, ∠MAD=∠NAD.
To Prove:  ΔAMN is an isosceles triangle. i.e any two sides in ΔAMN are
          equal.
Solution:  Now, In ΔADM and ΔADN
         ∠MAD=∠NAD   ...(1) (∵Given)
         AD=AD         ...(2) (∵common side)
         ∠ADM=∠ADN   ...(3) (∵Given)
         Hence, from equation (1),(2),(3) ΔADM ≅ ΔADN
                             ( ∵ ASA  congruence rule)
         ⇒ AM=AN
         Now, In Δ AMN
         AM=AN (∵ Proved)
         Hence, ΔAMN is an isosceles  triangle.