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IP属地:广东
1楼2012-01-29 04:46回复
![](http://data.artofproblemsolving.com/images/latex/c/3/0/c30a75c501b2198eb24e321ac1aaf800d2c785b3.gif)
has an inscribed circle
and
![](http://data.artofproblemsolving.com/images/latex/5/1/6/5164b05b5894374a2d32e0530428232eeb52c4d9.gif)
tangents
![](http://data.artofproblemsolving.com/images/latex/0/5/1/051212af7481e7874fa2eb9a5e14cecf3fe540f1.gif)
at
![](http://data.artofproblemsolving.com/images/latex/0/c/f/0cf399e39f2df6e2829ffb5cd243f5e4058a7a54.gif)
at
![](http://data.artofproblemsolving.com/images/latex/e/0/1/e0184adedf913b076626646d3f52c3b49c39ad6d.gif)
IP属地:广东
2楼2012-01-29 04:48
the inscribed circle at
![](http://data.artofproblemsolving.com/images/latex/a/3/6/a36a6718f54524d846894fb04b5b885b4e43e63b.gif)
Let
![](http://data.artofproblemsolving.com/images/latex/c/6/3/c63ae6dd4fc9f9dda66970e827d13f7c73fe841c.gif)
be the middle point of
![](http://data.artofproblemsolving.com/images/latex/3/f/1/3f12f1c08f58d70e355e83a54f01a76f82b9b96f.gif)
,then
![](http://data.artofproblemsolving.com/images/latex/c/6/3/c63ae6dd4fc9f9dda66970e827d13f7c73fe841c.gif)
is also the incentre of
![](http://data.artofproblemsolving.com/images/latex/c/3/0/c30a75c501b2198eb24e321ac1aaf800d2c785b3.gif)
IP属地:广东
3楼2012-01-29 04:49
IP属地:广东
4楼2012-01-29 04:53
IP属地:广东
5楼2012-01-29 05:03
IP属地:广东
8楼2012-01-29 13:52
证明:过点
![](http://data.artofproblemsolving.com/images/latex/a/3/6/a36a6718f54524d846894fb04b5b885b4e43e63b.gif)
作大圆的切线,交
![](http://data.artofproblemsolving.com/images/latex/2/7/4/27457e35dac8a9bc520179e1ba97bc6dcc537eff.gif)
于点
![](http://data.artofproblemsolving.com/images/latex/7/c/f/7cf184f4c67ad58283ecb19349720b0cae756829.gif)
,
![](http://data.artofproblemsolving.com/images/latex/5/9/6/596f0d1e7663099379943647166b1d6c4a2c4589.gif)
则
![](http://data.artofproblemsolving.com/images/latex/7/2/e/72ef85e173a5f69f7fe64c74641988e1b478dcba.gif)
是调和四边形,所以
![](http://data.artofproblemsolving.com/images/latex/8/0/7/8071e176d692beae950275aeb9e76f4833d9f1b7.gif)
另一方面 注意
![](http://data.artofproblemsolving.com/images/latex/e/1/c/e1cf0375192289abb06827e5ccd43f940fb7f710.gif)
由切割线定理有
IP属地:广东
9楼2012-01-29 13:56
IP属地:广东
10楼2012-01-29 13:57
IP属地:广东
11楼2012-01-29 13:57
由
![](http://data.artofproblemsolving.com/images/latex/2/6/f/26f51b3411e80ccaff40e7c548fa0cc538a73a7a.gif)
共圆知
![](http://data.artofproblemsolving.com/images/latex/5/2/0/520b4888a9fbf1c63e7cbd07878cc17d32cc9c86.gif)
分别共圆,且四边形
![](http://data.artofproblemsolving.com/images/latex/c/8/4/c84e44694084d3a57d6f1bf8608cb980826211bc.gif)
全部相似
So
![](http://data.artofproblemsolving.com/images/latex/e/2/4/e24d6ae54cebdbf78f7a5c42d3ea8859de963045.gif)
and 由调和四边形定义知
![](http://data.artofproblemsolving.com/images/latex/2/7/5/275adb1d856866a1dbd1b5444069567303fdeb11.gif)
所以有
![](http://data.artofproblemsolving.com/images/latex/0/4/2/0420627b8ed634b629d4afbcfa554f4ee5d42331.gif)
![](http://data.artofproblemsolving.com/images/latex/7/d/d/7ddb636c55eee8616cfc71f08d281d5e4256b630.gif)
IP属地:广东
12楼2012-01-29 13:58
IP属地:广东
13楼2012-01-29 13:59
IP属地:广东
14楼2012-01-29 14:00
这就是潘老师所说的为内切圆的基本性质之一
@维罗妮卡的微笑
求加精
![](http://static.tieba.baidu.com/tb/editor/images/face/i_f32.gif)
IP属地:广东
15楼2012-01-29 14:02
IP属地:广东
16楼2012-01-29 14:09
IP属地:美国
17楼2012-01-29 14:21
![](http://hiphotos.baidu.com/%BC%B8%D3%C3%C0%B4%B0%FC/pic/item/8559792ad40735fa835419089e510fb30e240899.jpg?v=tbs)
![](//tb1.bdstatic.com/tb/cms/PC%E7%AB%AF%E5%BA%95%E9%83%A8%E9%80%9A%E6%A0%8F%E5%BC%B9%E5%B1%821000x120.png)
![](//tb2.bdstatic.com/tb/static-spage/widget/fixed_bar/images/icon_close_1683259.png)