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1+2+3..+100 =(1+100)+(2+99)..(50+51) =101*50 =5050 求和公式 (首項+末項)*項數/2 首項(第一個數)=1 末項(最後一個數)=100 項數(多少個數)=100 所以(1+100)*100/2=5050
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