![](https://i.imgur.com/nl750yL.jpg)
Re: [中學] 國中幾何證明題
![](https://i.imgur.com/nl750yL.jpg)
: : 科學班甄選試題,求解
: REFER:
: http://mathworld.wolfram.com/OrthicTriangle.html
: ------------------------------------------------------------------------------
: DEF:
: a,b,c := 對邊
: R := △ABC外接圓半徑
: r := △DEF內接圓半徑
: ------------------------------------------------------------------------------
: PROPERTIES:
: △ABC的垂心是△DEF的內心
: 式一: △ABC=abc/4R
: 式二: △DEF=(abc*abs(cosAcosBcosC))/(2R)=(DE+EF+FD)*r/2
: 式三: r=2R*abs(cosAcosBcosC)
: ------------------------------------------------------------------------------
: PROOF:
: 由式二得 abc/2R,和式三一起 代入式一即可得到
: △ABC=R*(DE+EF+FD)/2
: ------------------------------------------------------------------------------
: 有現成的就偷懶不想自己證明...
想了個用三角函數的証明
首先利用共圓得到角度關係
http://imgur.com/bv3Z87a
![](https://i.imgur.com/bv3Z87a.jpg)
再定義一些角度
http://imgur.com/AJ7cPxu
![](https://i.imgur.com/AJ7cPxu.jpg)
接下來只要用正弦定理和角度關係做些代數運算
http://imgur.com/ycq3ndW
![](https://i.imgur.com/ycq3ndW.jpg)
![](https://i.imgur.com/dsJFJYc.jpg)
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