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Book I   Proposition 6
Then, since
    DB is equal to AC and BC is common,
    the two sides DB, BC are equal to the two sides AC, CB respectively and the angle DBC is equal to the angle ACB,
therefore
    the base DC is equal to the base AB and also
    the triangle DBC will be equal to the triangle ACB,
the less to the greater: which is absurd.

Therefore
    AB is not unequal to AC but rather
    AB is equal to AC.
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