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不妨設a1=1 a2=3依此類推容易得an-an-1=n那麼an-1-an-2=n-1 .a3-a2=3 a2-a1=2上式左邊和右邊相疊加an-a1=n+n-1+n-2+.+3+2=(n+n-1+n-2+.+3+2+1)-1=n*(n+1)/2-1則an=(n^2+n)/2
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