T H E E L E M E N T S	Book I   Proposition 15
	Similarly it can be proved that     the angles CEB, DEA are also equal. Therefore if two straight lines cut one another,     they make the vertical angles equal to one another. Being what it was required to prove. [From this it is manifest that, if two straight lines cut one another, they will make the angles at the point equal to four right angles.]
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