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S		 Book I   Proposition 13
But since each of the two pairs of angles, CBA, ABD and CBE, EBD, was proved equal to the three angles CBA, ABE, EBD;

and things which are equal to the same thing are also equal to one another; [C.N.1]
    therefore the angles CBA, ABD are also equal to the angles CBE, EBD.

But
    the angles CBE, EBD are two right angles;
therefore
    the angles DBA, EBD are also equal to two right angles.

Being what it was required to prove.
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