n×(n/n+1)=n-(n/n+1)(1-1/2)/10×(2-2/3)/9×(3-3/4)/8×...×(10-10/11)/1=(1×1/2)/10×(2×2/3)/9×(3×3/4)/8×...×(10×10/11)/1=1/2×2/3×3/4×...×10/11=1/11
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