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1×2+2×3+…+8×9太長了,就不抄全了.=1×(1+1)+2×(2+1)+…+8×(8+1)=1²;+2²;+…+8²;+1+2+…+8=8×9×17/6 +8×9/2=204+36=240利用公式:1+2+…+n=n(n+1)/21²;+2²;+…+n²;=n(n+1)(2…
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