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Book III   Proposition 16
For suppose it does not, but, if possible,
    let it fall within as CA,
and
    let DC be joined.

Since DA is equal to DC,
    the angle DAC is also equal to the angle ACD. [I.5]
But
    the angle DAC is right;
therefore
    the angle ACD is also right:
 
thus, in the triangle ACD, the two angles DAC, ACD are equal to two right angles: which is impossible. [I.17]
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