1/2+1/6+1/12+1/20+1/30+1/42
=ï¼1-1/2ï¼+ï¼1/2-1/3ï¼+ï¼1/3-1/4ï¼+ï¼1/4-1/5ï¼+ï¼1/5-1/6ï¼+ï¼1/6-1/7ï¼
=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7
=1-ï¼1/2-1/2ï¼-ï¼1/3-1/3ï¼-ï¼1/4-1/4ï¼-ï¼1/5-1/5ï¼-ï¼1/6-1/6ï¼-1/7
=1-0-0-0-0-0-1/7
=6/7
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