T H E E L E M E N T 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, ABD are also equal to two right angles. Being what it was required to prove. |
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