|Book III Proposition 37|
the angle DEF is right;
the angle DEF is also right.
And FB produced is a diameter; and
the straight line drawn at right angles to the diameter of a circle, form its extremity, touches the circle; [III.16, Por.]
DB touches the circle.
Similarly this can be proved to be the case even if the center be on AC.
Being what it was required to prove.
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