T H E E L E M E N T S |
Book II Proposition 4 | |
Then, since CF is parallel to AD, and BD has fallen on them, the exterior angle CGB is equal to the interior and oppostie angle ADB. [I.29] But the angle ADB is equal to the angle ABD, since the side BA is also equal to AD; [I.5] therefore the angle CGB is also equal to the angle GBC, so that the side BC is also equal to the side CG. [I.6] But CB is equal to GK, and CG to KB; [I.34] therefore GK is also equal to KB; therefore CGKB is equilateral. |
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