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Book I   Proposition 42
    Let BC be bisected at E
and
    let AE be joined;
on the straight line EC, and at the point E on it,
    let the angle CEF be constructed equal to the angle D; [I.23]
through A
    let AG be drawn parallel to EC, [I.31]
and through C
    let CG be drawn parallel to EF.

Then FECG is a parallelogram.
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