4. 求和:=__________________.
5. 数列的前n项和,则=________________.
6. 求和:=____________________.
7.求和:3+33+333+...+=_____________________. 学_ _ ]
8. 数列的前n项和为___________________.
9..
10.已知数列的前n项的和,又,求数列的前n项的和.
学_ _ _X_X_ ]