题目内容
一只袋子里装有红球和绿球,第一次从中摸出是红球和绿球的概率均为![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_ST/0.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_ST/1.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_ST/2.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_ST/3.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_ST/4.png)
(1)P1=______;P2=______;
(2)试写出Pn与Pn-1之间的关系式;______.
【答案】分析:(1)由一只袋子里装有红球和绿球,第一次从中摸出是红球和绿球的概率均为
,即可求得P1的值,又由如果上一次摸出是红球,则下一次摸出是红球的概率为
,绿球的概率为
;如果上一次摸出的是绿球,则下一次摸出的是红球的概率为
,绿球的概率为
,即可得P2=
P1+
(1-P1),继而求得P2的值;
(2)根据题意可得规律为:Pn=
Pn-1+
(1-Pn-1).
解答:解:(1)∵第一次从中摸出是红球和绿球的概率均为
,
∴P1=
;------(1分)
∵若第一次摸出是红球,则此时摸出是红球的概率为
,这种情况下的概率为:
×P1=
×
,
若第一次摸出是绿球,则此时摸出是红球的概率为
,这种情况下的概率为:
×(1-P1),
∴P2=
P1+
(1-P1)=
×
+
×
=
;----(3分)
(2)根据题意得:Pn=
Pn-1+
(1-Pn-1).--------(7分)
故答案为:(1)
,
;(2)
Pn-1+
(1-Pn-1).
点评:此题考查了概率公式的应用.此题属于规律性题目,难度比较大,解题的关键是根据题意得到规律:Pn=
Pn-1+
(1-Pn-1).
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/0.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/1.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/2.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/3.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/4.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/5.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/6.png)
(2)根据题意可得规律为:Pn=
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/7.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/8.png)
解答:解:(1)∵第一次从中摸出是红球和绿球的概率均为
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/9.png)
∴P1=
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/10.png)
∵若第一次摸出是红球,则此时摸出是红球的概率为
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/11.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/12.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/13.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/14.png)
若第一次摸出是绿球,则此时摸出是红球的概率为
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/15.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/16.png)
∴P2=
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/17.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/18.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/19.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/20.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/21.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/22.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/23.png)
(2)根据题意得:Pn=
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/24.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/25.png)
故答案为:(1)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/26.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/27.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/28.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/29.png)
点评:此题考查了概率公式的应用.此题属于规律性题目,难度比较大,解题的关键是根据题意得到规律:Pn=
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/30.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131022164832674814353/SYS201310221648326748143023_DA/31.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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