|Book II Proposition 9|
AC is equal to CE,
the angle EAC is also equal to the angle AEC.
the angle at C is right,
the remaining angles EAC, AEC are equal to one right angle. [I.32]
And they are equal; therefore
each of the angles CEA, CAE is half a right angle.
For the same reason
each of the angles CEB, EBC is also half a right angle;
the whole angle AEB is right.
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