巧算:(1⼀1+2)+(1⼀1+2+3)+…+(1⼀1+2+3+…+20)

求过程,谢谢
2025-01-01 11:48:45
推荐回答(1个)
回答1:

(1/1+2)+(1/1+2+3)+…+(1/1+2+3+…+20)

=2/2x3+2/3x4+……+2/20x21
=2x(1/2-1/3+1/3-1/4+……+1/20-1/21)
=2x(1/2-1/21)
=1-2/21
=19/21