H
E
E
L
E
M
E
N
T
S
Therefore
as DE is to BG, so is DE to EF. [VI.4]
But
as DE is to EF, so by hypothesis is AB to BC;
therefore
AB has the same ratio to each of the straight lines BC, BG; [V.11]
therefore
BC is equal to BG, [V.9]
so that
the angle at C is also equal to the angle BGC. [I.5]
