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