=1/3*1+1/4*2+1/5*3+1/6*4+……+1/（10+1)(10-1)
=1/2*[(1-1/3)+(1/2-1/4)+(1/3-1/5)+(1/4-1/6)+……+1/(10-1)-1/(10+1)]
=1/2*(1+1/2-1/10-1/(10+1))
=3/4-1/2*10-1/2(10+1)
=36/55