T H E E L E M E N T S	Book III   Proposition 31
	This is at once manifest. For, since     the angle contained by the straingt lines BA, AC is right,     the angle contained by the circumference ABC and the straight line AC is greater than a right angle. Again, since     the angle contained by the straight lines AC, CF is right,     the angle contained by the straight line CA and the circumference ADC is less than a right angle. Further being what it was required to prove.
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