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S		 Book IV   Proposition 2
Then, since a straight line AH touches the circle ABC, and from the point of contact at A the straight line AC is drawn across the circle, therefore
    the angle HAC is equal to the angle ABC in the alternate segment of the circle. [III.32]
But
    the angle HAC is equal to the angle DEF;
therefore
    the angle ACB is also equal to the angle DEF.

For the same reason
    the angle ACB is also equal to the angle DFE.
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