1／2＋1／6＋1／12．．．．．．＋1／9900
=(1-1/2)+(1/2-1/3)+(1/3-1/4)+...+(1/9899-1/9900)
=1-1/2+1/2-1/3+1/3.-1/9899+1/9899-1/9900
=1-1/9900
=9899/9900