Geometric Terms and Notation

Geometric Terms and Notation

Precise geometric language allows shapes, angles and relationships to be described unambiguously.

Lines and Angles

TermMeaning
Line segment \( AB \)Finite portion of a line between \( A \) and \( B \)
Parallel linesNever meet; marked with matching arrows
Perpendicular linesMeet at 90°; marked with a small square
\( \angle ABC \)Angle at vertex \( B \), from ray \( BA \) to ray \( BC \)
Acute / Right / Obtuse / Reflex\( 0°\text{–}90° \) / \( 90° \) / \( 90°\text{–}180° \) / \( 180°\text{–}360° \)

Triangle Conventions

  • Vertices \( A,B,C \) (capitals). Side \( a \) is opposite vertex \( A \).
  • Equal sides: tick marks. Equal angles: arcs. Right angle: small square — not an arc.
Equilateral Right-angled Labelled scalene A B C c b a

Polygon Terms

  • Interior angle sum: \( (n-2)\times 180° \). Each exterior angle of regular \( n \)-gon: \( 360°/n \).
  • Regular polygon: all sides and angles equal.

 Key Takeaways

  • \( \angle ABC \): vertex is the middle letter.
  • Equal sides: tick marks. Equal angles: arcs. Right angle: small square.
  • Side \( a \) is opposite vertex \( A \).
  • Interior angle sum: \( (n-2)\times180° \). Regular polygon exterior angle: \( 360°/n \).