Explanation:
a. if AB=AC then arc(AB)=arc(AC), then arc(AC)=140°;
b. if arc(AB)=arc(AC)=140°, then arc(DC)=360-[arc(AB)+arc(AC)]=360-280=80°;
c. if arc(BC)=80°, then m∠A=0.5*arc(BC)=40°;
d.; e. if AB=AC and m∠A=40°, then m∠B=m∠C=0.5(180-m∠A)=0.5(180-40)=70°.