题目内容
如图,在△ABC中,AB=5,BC=3,AC=4,动点E(与点A、C不重合)在AC边上,EF∥AB交BC于点F.![](http://thumb.1010pic.com/pic1/imagenew2/czsx/13/37153.jpg)
【小题1】当△ECF的面积与四边形EABF的面积相等时,求CE的长
【小题2】当△ECF的周长与四边形EABF的周长相等时,求CE的长
【小题3】试问在AB上是否存在点P,使得△EFP为等腰直角三角形?若不存在,请简要说明理由;若存在,请求出EF的长.
【小题1】CE=2
![](http://thumb.1010pic.com/pic1/imagenew2/czsx/14/40094.png)
【小题2】
![](http://thumb.1010pic.com/pic1/imagenew2/czsx/15/40095.png)
【小题3】在AB上存在点P.使△EFP为等腰直角三角形,此时EF=
![](http://thumb.1010pic.com/pic1/imagenew2/czsx/16/40096.png)
![](http://thumb.1010pic.com/pic1/imagenew2/czsx/17/40097.png)
解:(1)∵△ECF的面积与四边形EABF的面积相等,
∴S△ECF:S△ACB=1:2,
又∵EF∥AB,∴△ECF∽△ACB,
∴
![](http://thumb.1010pic.com/pic1/imagenew2/czsx/18/40098.png)
∴CE=2
![](http://thumb.1010pic.com/pic1/imagenew2/czsx/19/40099.png)
(2)设CE的长为x,
∵△ECF∽△ACB,
∴
![](http://thumb.1010pic.com/pic1/imagenew2/czsx/0/40100.png)
![](http://thumb.1010pic.com/pic1/imagenew2/czsx/1/40101.png)
由△ECF的周长与四边形EABF的周长相等,得:
x+EF+
![](http://thumb.1010pic.com/pic1/imagenew2/czsx/1/40101.png)
![](http://thumb.1010pic.com/pic1/imagenew2/czsx/1/40101.png)
解得x=
![](http://thumb.1010pic.com/pic1/imagenew2/czsx/4/40104.png)
![](http://thumb.1010pic.com/pic1/imagenew2/czsx/4/40104.png)
(3)△EFP为等腰直角三角形,有两种情况:
①如图1,假设∠PEF=90°,EP=EF
由AB=5,BC=3,AC=4,得∠C=90°
∴Rt△ACB斜边AB上高CD=
![](http://thumb.1010pic.com/pic1/imagenew2/czsx/6/40106.png)
![](http://thumb.1010pic.com/pic1/imagenew2/czsx/7/40107.png)
即
![](http://thumb.1010pic.com/pic1/imagenew2/czsx/8/40108.png)
解得x=
![](http://thumb.1010pic.com/pic1/imagenew2/czsx/9/40109.png)
![](http://thumb.1010pic.com/pic1/imagenew2/czsx/9/40109.png)
当∠EFP´=90°,EF=FP′时,同理可得EF=
![](http://thumb.1010pic.com/pic1/imagenew2/czsx/9/40109.png)
②如图2,假设∠EPF=90°,PE=PF时,点P到EF的距离为
![](http://thumb.1010pic.com/pic1/imagenew2/czsx/12/40112.png)
设EF=x,由△ECF∽△ACB,得:
![](http://thumb.1010pic.com/pic1/imagenew2/czsx/13/40113.png)
解得x=
![](http://thumb.1010pic.com/pic1/imagenew2/czsx/17/40097.png)
![](http://thumb.1010pic.com/pic1/imagenew2/czsx/17/40097.png)
综上所述,在AB上存在点P,使△EFP为等腰直角三角形,此时EF=
![](http://thumb.1010pic.com/pic1/imagenew2/czsx/9/40109.png)
![](http://thumb.1010pic.com/pic1/imagenew2/czsx/17/40097.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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