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Book III   Proposition 7
I say also that form the point F only two equal straight lines will fall on the circle ABCD, one on each side of the diameter AD. (continue)

For on the straight line EF, and at the point E on it,
    let the angle FEH be constructed equal to the angle GEF, [I.23] and let FH be joined.

Then, since GE is equal to EH, and EF is common,
    the two sides GE, EF are equal to the two sides HE, EF; and the angle GEF is equal to the angle HEF;
therefore
    the base FH is equal to the base FG. [I.4]
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