四分之二加十五分之二加三十五分之二加.加三百二十三分之二加三百九九分之二
=1/2+1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11+1/11-1/13+1/13-1/15+1/15-1/17+1/17-1/19+1/19-1/21
=1/2+1/3-1/21
=21/42+14/42-2/42
=33/42
=11/14