Proposition 15.
If two straight lines cut one another, they make the vertical angles equal to one another.
For let the straight lines
AB,
CD cut one another at the point
E;
I say that the angle
AEC is equal to the angle
DEB,
and the angle CEB to the angle AED.
For, since the straight line
AE stands
on the straight line
CD, making the angles
CEA,
AED,
the angles CEA, AED are equal to two right angles [I. 13]
Again, since the straight line
DE stands on the straight line
AB, making the angles
AED,
DEB,
the angles AED, DEB are equal to two right angles. [I. 13]
But the angles
CEA,
AED were also proved equal to two right angles;
therefore the angles CEA, AED are equal to the angles AED
DEB. [Post. 4 and C. N. 1] Let the angle AED be subtracted from each; therefore the remaining angle CEA is equal to the remaining angle BED. [C. N. 3]
Similarly it can be proved that the angles
CEB,
DEA are also equal.
Therefore etc. Q. E. D.
Porism.
[From this it is manifest that, if two straight lines cut one another, they will make the angles at the point of section equal to four right angles.
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