1/9*10+1/10*11+1/11*12+1/12*13+1/13*14+1/14*15+1/15*16+1/16*17+1/17*18
= 1/9 - 1/10 + 1/10 - 1/11 + 1/11 - 1/12 + ...+ 1/17 - 1/18
= 1/9 - 1/18
= 1/18