Question

1/10+1/15+1/21+...+1/120

Answer 1

=2/20+2/10+2/42+..+2/240=2.(1/20+1/30+1/42+...+1/240)

=2.(1/4.5+1/5.6+1/6.7+...+1/15.16)

=2.(1/4-1/5+1/5-1/6+1/6-1/7+...+1/15-1/16)=2.(1/4-1/16)

=3/8 nhé

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