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a2 - a1 = 1 a3 - a2 = 2 a4 - a3 = 3 . an - an-1 = n-1 將上式左右兩邊分別相加,得 an - a1 = 1+2+3+.+(n-1) 所以 an =(n-1)n/2 +a1 =(n-1)n/2
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