Question 1210173
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My question to Ikleyn is:

How do you know this:

The ratio of the area of triangle BAD' to the area of triangle BAC, equals the ratio of the length AD' to AC.

What theorem shows this is true? 

You didn't state any theorem that shows that. And it certainly is not obvious
to me.

Thanks for answering how you knew this to be true on your solution.

Edwin</pre>