How Can You Prove <)APB+<)CPD=180' in a Base-Quadrilateral Pyramid?

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There is a base-quadrilateraled pyramid ABCDS , which base is quadrilateral ABCD. Inscribed sphere is tangent in point P on the ABCD wall. Proof that

<)APB+<)CPD=180'


I don't like geometry and i really don't know how to start.
 
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I guess you are participating both Physics and Mathematics Polish Olympiad... Try to ask this question on the website www.matematyka.org
 
no I'm not
 
That's interesting, because only about 2-3 of your threads are not connected to problems given to solve on the first levels of those olympiads...
 

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