[分析] both open and closed sets

[paggei (XD)]

想請問，為什麼若a,b不在Ｑ中，則(a,b)∩Ｑ, [a,b]∩Ｑ在R中同時open且closed呢@@

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◆ From: 114.41.207.158
推 silvermare  :應該是在Ｑ同時open且closed吧?
→ silvermare  :      在Ｑ中
推 ab19881224  :推樓上~檢驗adherent point 是否在(a,b)之間吧?
推 silvermare  :簡易的作法:(a,b)open in R =>(a,b)∩Ｑ open in Ｑ
→ silvermare  :[a,b]closed in R =>[a,b]∩Ｑ closed in Q
→ silvermare  :(a,b)∩Ｑ = [a,b]∩Ｑ
→ paggei      :是的，在Q中沒錯，原來是我筆記抄錯了..囧
→ paggei      :感謝以上各位的回答@@3
→ paggei      :話說，我沒有學到adherent point 可能是還沒教吧@@