1/1*3+1/2*4+1/3*5+1/4*6+1/5*7+1/6*8=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]=1/2*[1+1/2-1/7-1/8]=1/2*(69/56)=69/112
10.9
