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Book VI   Proposition 5
Let ABC, DEF be two triangles having their sides proportional, so that,
    as AB is to BC, so is DE to EF,
    as BC is to CA, so is EF to FD, and further,
    as BA is to AC, so is ED to DF;

I say that
    the triangle ABC is equiangular with the triangle DEF, and they will have those angles equal which the corresponding sides subtend, namely
    the angle ABC to the angle DEF,
    the angle BCA to the angle EFD, and further
    the angle BAC to the angle EDF.
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