|Book VI Proposition 28|
And, since GF is parallel to DC,
the angle AFG is equal to the angle DCA;
the angle DAC is common to the two triangles ADC, AGF;
the triangle ADC is equiangular with the triangle AGF.
For the same reason
The triangle ACB is also equiangular with the triangle AFE,
the whole parallelogram ABCD is equiangular with the parallelogram EG.
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