2x1/4+3x1/5+4x1/6+5x1/7+6x1/8+7x1/9+8x1/10+9x1/11=2/4+3/5+4/6+5/7+6/8+7/9+8/10+9/11=77989/13860=5+8689/13860=5.62691197(691197循环)
