1\2+1\6+1\12+1\20+1\30+1\42
=(1-1/2)+(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+(1/5-1/6)+(1/6-`1/7)
=1-1/2+1/2-1/3+...+1/6-1/7
=1-1/7
=6/7;
如果本题有什么不明白可以追问,