第1个回答 2011-11-29
a.1/6+1/12=1/2-1/3+1/3-1/4=1/2-1/4=(1/4)
b.1/6+1/12+1/20=(1/2-1/3+1/3-1/4+1/4-1/5)-(1/2-1/5)=(3/10)
c.1/6+1/12+1/20+1/30+1/42=(1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6)=(1/2-1/6)=(1/3)
d.1/6+1/12+1/20+1/30+...+1/90=(1/2-1/3+1/3-1/4+1/4-1/5+……+1/9-1/10)=(1/2)-(1/10)=(2/5).