n分之1-n+1分之1=n*（n+1）分之1 利用上述规律计算2分之1,6分之1,12分之1,30分之1,42分之1的和.

1/2+1/6+1/12+1/20+1/30+1/42=1/(1x2)+1/(2x3)+1/(3x4)+1/(4x5)+1/(5x6)+1/(6x7)因为1/(n*(n+1))=1/n-1/(n+1)所以可知1/(1x2)=1/1-1/21/(2x3)=1/2-1/31/(3x4)=1/3-1/41/(4x5)=1/4-1/51/(5x6)=1/5-1/61/(6x7)=1/6-1/7...谢了