1/3+1/3*5+1/5*7+1/7*9+1/9*11=1-2/3+(1/3-1/5)/2+(1/5-1/7)/2+(1/7-1/9)/2=1-2/3+2/3-2/5+2/5-2/7+2/7-2/9=1-2/9=7/9
没看懂题目。