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S		 Book I   Proposition 40
Therefore
    the triangle GEF is equal to the triangle ABC,
for
    they are on equal bases BC, EF and in the same parallels BF, AG. [I.38]

But
    the triangle DEF is equal to the triangle ABC; (hypothesis)
therefore
    the triangle GEF is also equal to the triangle DEF, [C.N.1]
the less to the greater: which is impossible.
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