题目内容
在下列各式中,等号不成立的是( )A.
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_ST/0.png)
B.2x
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_ST/1.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_ST/2.png)
C.
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_ST/3.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_ST/4.png)
D.(x+2
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_ST/5.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_ST/6.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_ST/7.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_ST/8.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_ST/9.png)
【答案】分析:分别对每个选项进行运算,然后选出正确答案.
解答:解:(1)隐含条件a>0,∴
=
=-
,等式成立.
(2)∵x>0,∴2x
=![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_DA/4.png)
=
,等式成立.
(3)由表示形式可得a<0,故将a3开出来得,
=-a
,等式不成立.
(4)(x+2
+y)÷(
+
)=
÷(
+
)=
+
,等式成立.
故选C
点评:本题考查二次根式的化简,属于基础题,关键在于开根号时要注意字母的正负性.
解答:解:(1)隐含条件a>0,∴
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_DA/0.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_DA/1.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_DA/2.png)
(2)∵x>0,∴2x
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_DA/3.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_DA/4.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_DA/5.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_DA/6.png)
(3)由表示形式可得a<0,故将a3开出来得,
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_DA/7.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_DA/8.png)
(4)(x+2
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_DA/9.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_DA/10.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_DA/11.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_DA/12.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_DA/13.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_DA/14.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_DA/15.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103194137310145128/SYS201311031941373101451021_DA/16.png)
故选C
点评:本题考查二次根式的化简,属于基础题,关键在于开根号时要注意字母的正负性.
![](http://thumb2018.1010pic.com/images/loading.gif)
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