Find the rule: 1.5, 4.5, 13.5, (), 121.5 1,5,9,13,…… The tenth number is ()

Find the rule: 1.5, 4.5, 13.5, (), 121.5 1,5,9,13,…… The tenth number is ()

1.5,4.5,13.5,40.5,121.5
1,5,9,13,17,21,25,29,33,37