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S		 Book VI   Proposition 8
Therefore
    as BC which subtends the right angle in the triangle ABC is to BA which subtends the right angle in the triangle ABD,
    so is AB itself which subtends the angle at C in the triangle ABC to BD which subtends the equal angle BAD in the triangle ABD,
and
    so also is AC to AD which subtends the angle at B common to the two triangles. [VI.4]
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