T
H
E

E
L
E
M
E
N
T
S
Book VI   Proposition 24
And, since GF is parallel to DC,
    the angle AFG is equal to the angle DCA;
and
    the angle DAC is common to the two triangles ADC, AGF;
therefore
    the triangle ADC is equiangular with the triangle AGF.

For the same reason
    The triangle ACB is also equiangular with the triangle AFE,
and
    the whole parallelogram ABCD is equiangular with the parallelogram EG.
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