Ta có: 1/2+1/6+1/12+1/20+1/30+1/42
=1/(1*2)+1/(2*3)+1/(3*4)+1/(4*5)+1/(5*6)+1/(6*7)
=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7
=1-1/7
=6/7
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=1/1.2+1/2.3+1/3.4+1/4.5+1/5.6+1/6.7
=1/1-1/7
=6/7
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