△AOCと△BODで
仮定から OA＝OB ...①
AC平行DBから ∠OAC＝∠OBD ...②
対頂角は等しいから ∠AOC＝∠BOD ...③
①②③より 1組の辺とその両端の角がそれぞれ等しいから
△AOC＝△BOD
よって OC＝OD
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