1*3分之2+3*5分之2+5*7分之2+7*9分之2=1-1/3+1/3-1/5+1/5-1/7+1/7-1/9=1-1/9=8/9
=2/(3*1)+2/(5*3)+2/(7*5)+2/(9*7)=1-1/3+1/3-1/5+1/5-1/7+1/7-1/9=1-1/9=8/9
