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1/3*5+1/5*7+1/7*9+1/9*11+1/11*13 =1/2×(1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11+1/11-1/13) =1/2×(1/3-1/13) =1/2×10/39 =5/39.
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