提取=(1/5+1/7+1/9+1/11)*(1/7+1/9+1/11)+(1/5+1/7+1/9+1/11)*1/13-(1/5+1/7+1/9+1/11)*(1/7+1/9+1/11)-1/13*(1/7+1/9+1/11)=(1/5+1/7+1/9+1/11)*1/13-1/13*(1/7+1/9+1/11)=1/13*(1/5+1/7+1/9+1/11-1/7+1/9+1/11）=1/13*1/5=1/65