找规律,第n项.

问题描述:

找规律,第n项.
(1)2,4,6,8.,( )第n项
(2)3,5,7,9.,( )第n项
(4)1又1/2,2又2/3,3又3/4.( )第n项

1 2n
2 2n+1
4 3=n又n+1分之n