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1 × 2 / 1 + 2 × 3 / 1 + 3 × 4 / 1 + 4 × 5 / 1 +... + 39 × 40 / 1 =1-1/2+1/2-1/3+1/3-1/4+…… +1/39-1/40 =1-1/40 =39/40
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