（1/2）÷3=1/6=(1/2)－(1/3) 同样(1/3)÷4=(1/3)－(1/4) (1/99)÷100=(1/99)－(1/100)
所以原式=(1/2)+(1/2)－(1/3)+(1/3)－(1/4)+.+(1/99)－(1/100)
=1－(1/100)
=99/100