题目内容
阅读下面解题过程,判断是否正确.若正确,则在题后的横线上写“正确”两字;若错误,则在题后的横线上写上开始出现错误的那一步的序号,并写出正确的解题过程.题:已知a=20,b=15,求
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_ST/0.png)
解:原式=
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_ST/1.png)
=样
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_ST/2.png)
=
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_ST/3.png)
=
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_ST/4.png)
=
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_ST/5.png)
当a=20,b=15时,原式=35
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_ST/6.png)
答案:③
【答案】分析:因为
=
,所以下一步要去绝对值,必须知道a-
b,
a-b的符号,因为a=20,b=15,所以a-
b>0,
a-b<0,b=15,所以a-
b>0,
a-b<0,所以上式=
=
,再把a=20,b=15代入
计算即可.
解答:第③步.
证明:原式=
,
=
,
=
,
∵a=20,b=15,所以a-
b>0,
a-b<0,
∴上式=
,
=
,
∴当a=20,b=15时,原式=15
.
点评:本题考查了
的化简,若a≥0,则
=a,若a<0则
=-a.
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_DA/0.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_DA/1.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_DA/2.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_DA/3.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_DA/4.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_DA/5.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_DA/6.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_DA/7.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_DA/8.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_DA/9.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_DA/10.png)
解答:第③步.
证明:原式=
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_DA/11.png)
=
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_DA/12.png)
=
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_DA/13.png)
∵a=20,b=15,所以a-
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_DA/14.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_DA/15.png)
∴上式=
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_DA/16.png)
=
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_DA/17.png)
∴当a=20,b=15时,原式=15
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_DA/18.png)
点评:本题考查了
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_DA/19.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_DA/20.png)
![](http://thumb.1010pic.com/pic6/res/czsx/web/STSource/20131103195331787308880/SYS201311031953317873088020_DA/21.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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