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Book I   Proposition 25
It was proved that the angle BAC is neither less than nor equal to the angle EDF; therefore
    the angle BAC is greater than the angle EDF.

Therefore
    if two triangles ABC, DEF have the two sides AB, AC equal to the two sides ED, DF respectively, and the base BC greater than the base EF, the angle BAC is also greater than the angle EDF.

Being what it was required to prove.
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