H
E
E
L
E
M
E
N
T
S
therefore
FC is perpendicular to KL; [III.18]
therefore
each of the angles at C is right.
For the same reason
the angles at the points B, D are also right.
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T H E E L E M E N T S |
Book IV Proposition 12 | |
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Then, since the straight line KL touches the circle ABCDE at C,
and FC is joined from the center F to the point of contact C, therefore FC is perpendicular to KL; [III.18] therefore each of the angles at C is right. For the same reason the angles at the points B, D are also right. |
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