In the figure below, ∠ACB=40∘ and BC=AC. Also, E is the mid-point of AB. ∠ACE is equal to
In △AEC and △BEC,
∠A=∠B [Angles corresponding to equal sides]
AE=BE [E is the mid-point of AB]
BC=AC [Given]
⇒△AEC≅△BEC [SAS congruency]
⇒∠BCE=∠ACE [CPCTE]
∠ACB=∠BCE+∠ACE=2∠ACE
⇒∠ACE=∠ACB2=40∘2=20∘