■ Points, Lines, and Angles

Undefined Terms

Geometry starts with three undefined terms: point (location, no size), line (infinite in both directions), plane (flat surface extending infinitely). All other geometric figures are defined from these.

Angle Classification

• Acute: 0° < θ < 90°
• Right: θ = 90°
• Obtuse: 90° < θ < 180°
• Straight: θ = 180°
• Reflex: 180° < θ < 360°

Angle Relationships

• Complementary: two angles summing to 90°
• Supplementary: two angles summing to 180°
• Vertical angles: formed by two intersecting lines; they are congruent
• Linear pair: adjacent angles forming a straight line; supplementary