T H E E L E M E N T S |
Book II Proposition 4 | |
I say that it is also right-angled. For, since CG is parallel to BK, the angles KBC, GCB are equal to two right angles. [I.29] But the angle KBC is right; therefore the angle BCG is also right, so that the opposite angles CGK, GKB are also right. [I.34] Therefore CGKB is right-angled. |
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