题目有误，应为：5/1*2*3*4+7/2*3*4*5+9/3*4*5*6+....+17/7*8*9*10 =1/1*3-1/2*4+1/2*4-1/3*5+1/3*5-1/4*6+....+1/7*9-1/8*10= 1/1*3-1/8*10=1/3-1/80=77/240
