Prove that if angle C = angle D, then angle A = angle E.
https://homework.russianschool.com/resource?key=54101jiop7nbb
Since DBE and CBA are straight lines, angle(DBA) = angle(CBE) because they are vertical angles.
Somce there are 180o in every triangle:
angle(D) + angle(A) + angle(DBA) = 180o = angle(C) + angle(E) + angle(CBE)
angle(D) + angle(A) + angle(DBA) = angle(C) + angle(E) + angle(CBE)
angle(A) = angle(E) (by sutbraction)