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S		 Book IV   Proposition 15
    The triangle EGD is equilateral;
and therefore
    its three angles EGD, GDE, DEG are equal to one another, inasmush as, in isosceles triangles, the angles at the base are equal to one another. [I.5]

And the three angles of the triangle are equal to two right angles; [I.32] therefore
    the angle EGD is one-third of two right angles.

Similarly,
    the angle DGC can also be proved to be one-third of two right angles.
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