①an =2 (this is a constant sequence)

(2) Without looking at symbols, we found such a rule.

1/8= 1/2? , 1/27 = 1/3? , 1/64= 1/4? ……

Look at these symbols again. The first one is negative and the second one is positive. ...................................................................................................................................................................

∴an=(- 1)^n ×﹙ 1/n？ ﹚

③ The nth ∴ of molecules 3, 4, 5, 6 ... is n+2.

Look at the denominators 5, 8, 1 1 ... in turn.

∴an=(n+2)/﹙3n+2﹚