(1/3+1/5+1/7)*(1/5+1/7+1/9)-(1/3+1/5+1/7+1/9)*(1/5+1/7)=?

问题描述:

(1/3+1/5+1/7)*(1/5+1/7+1/9)-(1/3+1/5+1/7+1/9)*(1/5+1/7)=?

原式=(1/3+1/5+1/7)*(1/5+1/7+1/9)-(1/3+1/5+1/7)(1/5+1/7)-1/9*(1/5+1/7)=(1/3+1/5+1/7)*(1/5+1/7+1/9-1/5-1/7)-1/9(1/5+1/7)=(1/3+1/5+1/7)*(1/9)-1/9(1/5+1/7)=1/9*(1/3+1/5+1/7-1/5-1/7)=1/9*1/3=1...