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I wonder what's the theorem of geometry on the green facepage in the following website:

https://www.google.co.jp/search?q=a+course+of+pure+mathematics+by+g.+h.+hardy+geometry&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiu05_s7OffAhVbeXAKHWVICuQQ_AUIDigB&biw=1536&bih=750#imgrc=hBZJcysX7IsL-M:&spf=1547282635834

It seems NOT simson's theorem on a circle. Please teach me.

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closed as off-topic by Mark Wildon, Wojowu, Alexey Ustinov, David Loeffler, j.c. Jan 12 at 14:56

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