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S		 Book I   Proposition 25
For, if not, it is either equal to the angle DEF or less.

Now the angle BAC is not equal to the angle EDF, for then
    the base BC would also have been equal to the base EF; [I.4]
but it is not;
therefore the angle BAC is not equal to the angle EDF.

Neither is the angle BAC less than the angle EDF, for then
    the base BC would also have been less than the base EF; [I.24]
but it is not;
therefore the angle BAC is not less than the angle EDF.
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