T
H
E

E
L
E
M
E
N
T
S
Book VI   Proposition 20
For, if BD be again joined, since,
    as CA is to AD,
    so is EA to AB,
while,
    as CA is to AD,
    so is the triangle ABC to the triangle BAD,
and
    as EA is to AB,
    so is the triangle EAD to the triangle BAD, [VI.I]
therefore,
    as the triangle ABC is to the triangle BAD,
    so is the triangle EAD to the triangle BAD; [V.11]
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