1-6/5+12/7-20/9+30/11-42/13+56/15-72/17
=1-(2/1+3/1)+(3/1+4/1)-(4/1+5/4)+(5/1+6/1)-(6/1+7/1)+(7/1+8/1)-(8/1+9/1)
=1-2/1-3/1+3/1+4/1-4/1-5/4+5/1+6/1-6/1-7/1+7/1+8/1-8/1-9/1
=1-2/1-9/1
=18/7