1/歼册中6+1/12+1/20+1/30+1/42+1/56+1/72+1/90
=(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+(1/5-1/6)+(1/6-1/7)+(1/7-1/8)+(1/8-1/9)+(1/9-1/10)
=1/2-1/3+1/3-1/4+1/氏山4-1/姿举5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10
=1/2-1/10
=4/10
=2/5
=(2*3)/1+(3*4)/陆辩1+......+(9*10)/1
=1/2-1/3+1/3-1/伍拍4+1/4-...-1/9+1/早橘缺9-1/10
=1/2-1/10
=2/5
1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90
=1/2+(1/2-1/3)+(1/洞燃友3-1/4)+(1/4-1/5)+(1/5-1/6)+(1/段咐6-1/7)+(1/7-1/8)+(1/8-1/9)+(1/9-1/纳槐10)
=1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10
=1/2+1/2-1/10
=1/2+4/10
=1/2+2/5
=9/10