1/2+1/6+1/12+1/20+.+1/380+1/420 等于?

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1/2+1/6+1/12+1/20+.+1/380+1/420等于?1/2+1/6+1/12+1/20+.+1/380+1/420等于?1/2+1/6+1/12+1/20+.+1/380+1/420等

1/2+1/6+1/12+1/20+.+1/380+1/420 等于?
1/2+1/6+1/12+1/20+.+1/380+1/420 等于?

1/2+1/6+1/12+1/20+.+1/380+1/420 等于?
1/2+1/6+1/12+1/20+.+1/380+1/420
=1/(1×2)+1/(2×3)+1/(3×4)+1/(4×5)+.+1/(19×20)+1/(20×21)
=1-1/2+1/2-1/3+1/3-1/4+...+1/20-1/21
=1-1/21
=20/21