2/1+6/1+12/1+20/1+30/1+42/1+56/1+72/1+90/1
=1-2/1+2/1-3/1+3/1-4/1+4/1-5/1+5/1-6/1+6/1-7/1+7/1-8/1+8/1-9/1+9/1-10/1
=1-10/1
=10/9