1/3 1/2 5/9 7/12 3/5 11/18·····,
即为1/3 3/6 5/9 7/12 9/15 11/18·····,
分子成1为首项,2为公差的等差数列
an=2n-1
分母成3为首项,3为公差的等差数列
bn=3n
则这组数中的第40个数是c40=a40/b40=79/120