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Book III   Proposition 37
Now, since DE touches the circle ABC, and DCA cuts it,
    the rectangle AD, DC is equal to the square on DE. [III.36]
But
    the rectangle AD, DC was also equal to the square on DB;
therefore
    the square on DE is equal to the square on DB;
therefore DE is equal to DB.

And FE is equal to FB; therefore
    the two sides DE, EF are equal to the two sides DB, FB; and FD is the common base of the triangles;
therefore
    the angle DEF is equal to the angle DBF. [I.8]
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