（1-1/2）(1-1/3)(1-1/4)…(1-1/998)(1-1/999)(1-1/1000) =(1/2)*(2/3)*(3/4)*...*(999/1000)=1/1000(2000-1990)+(1980-1970)+…+(20-10)=10*100=10002000-1990+1980-1970+…+20-10除以（1-1/2）(1-1/3)(1-1/4)…(1-...