T H E E L E M E N T S |
Book III Proposition 19 | |
For suppose it is not, but, if possible, let F be the center, and let CF be joined. Since a straight line DE touches the circle ABC, and FC has been joined from the center to the point of contact, FC is perpendicular to DE; [III.18] therefore the angle FCE is right. But The angle ACE is also right; therefore the angle FCE is equal to the angle ACE, |
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