T
H
E

E
L
E
M
E
N
T
S
Book II   Proposition 10
Then since
    EC is equal to CE,
    the angle EAC is also equal to the angle AEC; [I.5]
and
    the angle at C is right;
therefore
    each of the angles EAC, ACE is half a right angle. [I.32]

For the same reason
    each of the angles CEB, EBC is also half a right angle, therefore
    the angle AEB is right.
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