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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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