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I cannot remember what the theorem is in which the following happens:

Given two lines l and m which intersect each other, let H be the point of intersection.

Let A and B be points on the line l such that AHB are colinear. And let C and D be points on the line m such that CHD are colinear.

Now what is the theorem/lemma/corollary which states that when two such lines intersect in such a way that

angle AHC = angle BHD

and

angle AHD = angle BHC ?

I need to quote it for a proof that I am doing. I can't remember for my life. And I can't seem to find it in my notes/text. It's not a big problem, I would just like to quote it properly.

Any help is appreciated. Thankyou.

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# Homework Help: Geometry: What Theorem Is This?

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