Answer:
In ΔECB ,
∠ECD = ∠EBC + ∠BEC (∵ Exterior Angle Property of a triangle)
⇒ ∠BEC = ∠ECD - ∠EBC ................. eqn.1
In ΔABC ,
∠ACD = ∠BAC + ∠ABC (∵ Exterior Angle Property of a triangle )
⇒ 2∠ECD = ∠BAC + 2∠EBC
⇒ ∠BAC = 2∠ECD - 2∠EBC = 2(∠ECD - ∠EBC)
( Putting eqn.1 here )
⇒ ∠BAC = 2∠BEC (∵∠BEC = ∠ECD - ∠EBC)
