练习：求数列
|
令S=
+2 2
+4 22
+…+6 23 2n 2n
则
S=1 2
+2 22
+…+4 23
+2n-2 2n 2n 2n+1
两式相减可得，
S=1+2(1 2
+1 22
+…+1 23
)-1 2n
=1+2×2n 2n+1
-
(1-1 4
)1 2n-1 1- 1 2
=2-n 2n
-1 2n-1 n 2n
∴s=4-2+n 2n-1
| 从方框中选择恰当的句子完成对话。 | |
B: She is a shop assistant. A: Do you want to be a shop assistant? B: (2) ______ A: Why don't you want to be a shop assistant? B: (3) ______ A: Then (4) _______ B: I want to be a policewoman. A: A policewoman? Why? B: (5) ______ What about you, Jim? A: Oh, I want to be a teacher because I love school and l love students. |