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**Al-Allo****Member**- Registered: 2012-08-23
- Posts: 324

We have the following statement by Euclid : "I say further that the angle of the semicircle contained by the straight line BA and the circumference CHA is greater than any acute rectilinear angle, and the remaining angle contained by the circumference CHA and the straight line AE is less than any acute rectilinear angle." (Book 3 proposition 16)

The figure is in this link : http://aleph0.clarku.edu/~djoyce/java/elements/bookIII/propIII16.html

So, I was wondering, could somebody give me an example of where it is true ? For this proposition to be true, must the given straight angle be the same for both curved angles ? Or different?

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