1/1*3=(1-1/3)/2
1/3*5=(1/3-1/5)/2
1/5*7=(1/5-1/7)/2
1/7*9=(1/7-1/9)/2
1/1*3+1/3*5+1/5*7+1/7*9=(1-1/3+1/3-1/5+1/5-1/7+1/7-1/9)/2=(1-1/9)/2=4/9