One third plus one sixth plus one tenth plus one fifteenth plus one twenty-one plus one twenty-eight plus one thirty-six plus one forty-five One fifty-five and one sixty-six is equal to?
Original formula = (1 / 3 + 1 / 6) + (1 / 10 + 1 / 15) + ((1 / 21 + 1 / 28) + (1 / 45 + 1 / 36) + (1 / 55 + 1 / 66)
=3/6+5/30+7/84+9/180+11/330
=1/2+1/6+1/12+1/20+1/30
=1/2+1/6+1/12+(1/20+1/30)
=1/2+1/6+1/12+1/12
=1/2+1/6+1/6
=1/2+1/3=5/6
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