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# 1: Lines, Angles, and Triangles

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• 1.1: Lines
Geometry (from Greek words meaning earth-measure) originally developed as a means of surveying land areas, In its simplest form, it is a study of figures that can be drawn on a perfectly smooth flat surface, or plane. It is this plane geometry which we will study in this bock and which serves as a foundation for trigonometry, solid and analytic geometry, and calculus.
• 1.2: Angles
An angle is the figure formed by two rays with a common end point, The two rays are called the sides of the angle and the common end point is called the vertex of the angle.
• 1.3: Angle Classifications
• 1.4: Parallel Lines
Two lines are parallel if they do not meet, no matter how far they are extended.
• 1.5: Triangles
A triangle is formed when three straight line segments bound a portion of the plane, The line segments are called the sides of the triangle. A point where two sides meet is called a vertex of the triangle, and the angle formed is called an angle of the triangle, The symbol for triangle is △ .
• 1.6: Triangle CIassifications

Thumbnail: Angles A and B are adjacent. (Public Domain; Limaner via Wikipedia)