1 / 2 = 1-1 / 2, 1 / 6 = 1 / 2-1 / 3, 1 / 12 = 1 / 3-1 / 4/

1/6+1/12+1/20
=1/2-1/3+1/3-1/4+1/4-1/5
=1/2-1/5
=3/10

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