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原式=(1/2+1/3)/4+(1/3+1/4)/5+...+(1/8+1/9)/10+(1/9+1/10)/11
=1/2*1/4+1/3*1/4+1/3*1/5+1/4*1/5+...+1/8*1/10+1/9*1/10+1/9*1/11+1/10*1/11
=1/2*(1/2-1/4+1/3-1/5+...+1/8-1/10+1/9-1/11)+1/3-1/4+1/4-1/5+...+1/9-1/10+1/10-1/11
=1/2*(1/2+1/3-1/10-1/11)+1/3-1/11
=1/2*(1/2+1-1/11-3/11)=31/55
=1/2*1/4+1/3*1/4+1/3*1/5+1/4*1/5+...+1/8*1/10+1/9*1/10+1/9*1/11+1/10*1/11
=1/2*(1/2-1/4+1/3-1/5+...+1/8-1/10+1/9-1/11)+1/3-1/4+1/4-1/5+...+1/9-1/10+1/10-1/11
=1/2*(1/2+1/3-1/10-1/11)+1/3-1/11
=1/2*(1/2+1-1/11-3/11)=31/55
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