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