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(^2)表示平方 因為S n-1=(n-1)^2 +n-1-1 S n = n^2 +n-1 所以S n - S n-1 = an n^2 +n-1-n^2 +2n-1+2-n=an 2n=an 所以an=2n 為等差數列
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