T
H
E

E
L
E
M
E
N
T
S
Book II   Proposition 10
And, since
    the angle EBC is half a right angle,
    the angle DBG is also half a right angle. [I.15]
But
    the angle BDG is also right, for it is equal to the angle DCE, they being alternate; [I.29]
therefore
    the remaining angle DGB is half a right angle; [I.32]
therefore
    the angle DGB is equal to the angle DBG,
so that
    the side BD is also equal to the side GD. [I.6]
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