1/1*3+1/2*4+1/3*5+1/4*6+1/5*7+1/6*8+1/7*9+1/8*10+1/9*11
=(1/2)*[1-1/3+1/2-1/4+1/3-1/5+1/4-1/6+1/5-1/7+1/6-1/8+1/7-1/9+1/8-1/10+1/9-1/11]
=(1/2)*[1-1/11+1/2-1/10]
=(1/2)*[165/110-10/110-11/110]
=(1/2)*72/55
=36/55