Proposition 13.
If a straight line set up on a straight line make angles, it will make either two right angles or angles equal to two right angles.
For let any straight line
AB set up on the straight line
CD make the angles
CBA,
ABD;
I say that the angles
CBA,
ABD are either two right angles or equal to two right angles.
Now, if the angle
CBA is equal to
the angle
ABD,
they are two right angles. [Def. 10]
But, if not, let
BE be drawn from the point
B at right angles to
CD; [
I. 11]
therefore the angles CBE, EBD are two right angles.
Then, since the angle
CBE is equal to the two angles
CBA,
ABE,
let the angle EBD be added to each; therefore the angles CBE, EBD are equal to the three angles CBA, ABE, EBD. [C. N. 2]
Again, since the angle
DBA is equal to the two angles
DBE,
EBA,
let the angle ABC be added to each; therefore the angles DBA. ABC are equal to the three angles DBE, EBA, ABC. [C. N. 2]
But the angles
CBE,
EBD were also proved equal to the same three angles;
and things which are equal to the same thing are also equal to one another; [C. N. 1] therefore the angles CBE, EBD are also equal to the angles DBA, ABC.
But the angles
CBE,
EBD are two right angles;
therefore the angles DBA, ABC are also equal to two right angles.
Therefore etc.
Q. E. D.
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