7/6+13/12+21/20+31/30+… +421/420
The original formula = 1 + 1 / (2 * 3) + 1 + 1 / (3 * 4) + 1 + 1 / (4 * 5) +. + 1 + 1 / (20 * 21)
=19 + (1/2 -1/3) + (1/3 -1/4) +(1/4 -1/5) + .+ (1/20 -1/21)
= 19 + 1/2 -1/21
=19 and 19 / 42
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