憑經驗，小球隻能在單位圓的下半圓周上自由滑動。這裏AB段在第一像限，是上半圓。

來源: wxcfan123 於 2021-06-20 09:35:22 [舊帖] [給我悄悄話] 本文已被閱讀：次

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