1+2+3+...+9=45
1/6+1/12+...+1/90
=1/2-1/3+1/3-1/4+...+1/9-1/10
=1/2-1/10
=2/5
1+2又6/1+3又12/1+4又20/1+5又30/1+6又42/1+7又56/1+8又72/1+9又90/1=45又5/2