①99又34分之33除以17分之1=(99X34+33)/34X17=[(100-1)X34+33]/2=(34X100-34+33)/2=3399/2=1699.5②7/6+13/12+21/20+31/30+...+421/420=1+1/6+1+1/12+...+1+1/420=1+1/(2X3)+1+1/(3X4)+...+1+1/(20X21)=1X19+1/2-1/3...