|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.
the angles CBE, EBD are two right angles;
the angles DBA, ABD are also equal to two right angles.
Being what it was required to prove.
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