急,1998/1*2+1998/2*3+1998/3*4+1998/4*5+1998/5*6,
问题描述：
急,1998/1*2+1998/2*3+1998/3*4+1998/4*5+1998/5*6,
答
1998/1*2+1998/2*3+1998/3*4+1998/4*5+1998/5*6=1998*(1/1*2+1/2*3+1/3*4+1/4*5+1/5*6)=1998*(1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6)=1998*(1-1/6)=1998*5/6=1665