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Book VI   Proposition 7
But, by hypothesis,
    the angle at C is less than a right angle;
therefore
    the angle BGC is also less than a right angle;
so that
    the angle AGB adjacent to it is greater than a right angle. [I.13]
And
    it was proved equal to the angle at F;
therefore
    the angle at F is greater than a right angle.
But
    it is by hypothesis less than a right angle: which is abusrd.

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