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Re:公式的证明,不知对不对

Posted by chpzcc at 2012-06-04 14:24:30 on Problem 1031
In Reply To:终于看懂题目了,附题目翻译 Posted by:chpzcc at 2012-06-04 14:07:46
设dl的一个端点为点A,一个端点为点B,点O(台灯)作垂直于该栅栏的线,交栅栏于点D,如下图:

    ————D——A—-—B—————  OA请自行想象...
	    |         /
            |        /
            |       / 
            |      /
            |     /
            |    /
            |   /
            |  /
            | /
            |/
            O

有OB=r, 那么OD=r*cosa
设OA与OB的夹角为da,则,
dl
=AB
=BD-AD
=OD*(tana-tan(a-da))
=r*(sina-cosa*(tana-tanda)/(1+tana*tanda))      //tan(a-b)公式
=r*(sina*tana*tanda+cosa*tanda)/(1+tana*tanda)  //通分
=r*tanda/(cosa+sina*tanda)                      //上下同乘cosa
=r*sinda/(cosa*cosda+sina*sinda)                //上下同乘cosda
=r*sinda/cos(a-da)
=r*da/cos(a-da)                                 //sinda=da                                
=r*da/cosa                                      //这步不知道是不是有点问题

所以
dI
=I0*|cosa|*dl*h
=k/r*|cosa|*r*da/cosa*h
=k*h*da

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