=1/1*3+1/3*5+1/5*7+1/7*9+1/9*11+1/11*13
=1/2*[(1-1/3)+(1/3-1/5)+(1/5-1/7)+(1/7-1/9)+(1/9-1/11)+(1/11-1/13)}
=1/2*[1-1/13]
=6/13