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S		 Book VI   Proposition 5
Therefore the angle EDF is also equal to the angle GEF, and the angle EDF to the angle EGF.

And, since
    the angle FED is equal to the angle GEF,
while
    the angle GEF is equal to the angle ABC,
therefore
    the angle ABC is also equal to the angle DEF.
For the same reason
    the angle ACB is also equal to the angle DFE,
and further,
    the angle at A to the angle at D;
therefore
    the triangle ABC is equiangular with the triangle DEF.
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