Allow ABC to be an
equilateral triangle so AB = AC = BC. Let X be the midpoint of BC, AX is the
median of BC, and BX = CX. Look at triangles BAX and CAX. Obviously, AX = AX since
AB = AC and BX = CX, subsequently by SSS Congruence Test, we have that
triangles BAX and CAX are congruent. Therefore, corresponding angles are
congruent so <ABX = <ACX. Because <ABX and <ABC are the same angle,
they are obviously congruent, and then <ABX = <ABC. Likewise, <ACX and
<ACB are the same angles so <ACX = <ACB. Then, <ABC = <ACB. Please
note that <ABC = <B and <ACB = <C. Therefore, <B = <C.
At present, disregard AX and let Y be the midpoint of AC.
Then BY is the median of AC. Then AY = CY. Evidently, BY = BY. Because AB = BC, we see that triangles ABY
and CBY are congruent by SSS. Then corresponding sames are congruent so <BAY
= <BCY. Since <BAY and <BAC are the same angles then <BAY =
<BAC. Similarly, <BCY = <BCA since they are the same angles. Then
<BAC = <BCA. Note that <BAC = <A and <BCA = <C. Therefore,
<A = <C.
Thus, <A = <C = <B. So all the angles are congruent.
