[b][url=https://mathcs.clarku.edu/~djoyce/java/elements/bookI/defI15.html]Definition 15[/url].[br][/b]A [i]circle[/i] is a plane figure contained by one line such that all the straight lines falling upon it from one point among those lying within the figure equal one another.[b][br][br][b][url=https://mathcs.clarku.edu/~djoyce/java/elements/bookI/defI15.html]Definition 16[/url].[/b][br][/b]And the point is called the [i]center[/i] of the circle.[br][br][br]
[b][url=https://mathcs.clarku.edu/~djoyce/java/elements/bookI/defI15.html]Definition 17[/url].[br][/b]A [i]diameter[/i] of the circle is any straight line drawn through the center and terminated in both directions by the circumference of the circle, and such a straight line also bisects the circle.[br][br][b][url=https://mathcs.clarku.edu/~djoyce/java/elements/bookI/defI15.html]Definition 18[/url].[br][/b]A [i]semicircle[/i] is the figure contained by the diameter and the circumference cut off by it. And the center of the semicircle is the same as that of the circle.
[b][url=https://mathcs.clarku.edu/~djoyce/java/elements/bookI/defI10.html]Definition 10[/url].[br][/b]When a straight line standing on a straight line makes the adjacent angles equal to one another, each of the equal angles is [i]right,[/i] and the straight line standing on the other is called a [i]perpendicular[/i] to that on which it stands.