Basic Geometric Terms

Definition Example Point – an exact location in space. A point has no A (read “point A”) dimension.

Line – a collection of points along a straight path C B that extends endlessly in both directions. ̼̽žʌʌʌʌľ (read “line CB”)

A B Line Segment – a part of a line having two endpoints. Ŭ̻̼ŬŬŬ (read “line segment ̻̼ ”) The length of Ŭ̻̼ŬŬŬ is denoted ̻̼ .

C D Ray – a part of a line having only one endpoint. ̽̾ʌʌʌʌʌľ (read “ray ̽̾ ”) The endpoint is always the first letter.

C

Vertex Angle – consists of two rays that have a common endpoint called the vertex of the angle. A B ǹ̻̼̽ (read “angle ̻̼̽ ”) The vertex is always the middle letter. ǹ̻̼̽ can also be written as ǹ̼̻̽ or just ǹ̼.

B C Plane – a flat surface that extends endlessly in all directions. A D Plane ̻̼̽̾

Straight Angle – an angle whose measure is 180°. A B C ǹ̻̼̽ is a straight angle.

D Symbol for Right Angle – an angle whose measure is 90°. right angle

E F ǹ̾̿̀ is a right angle. X Acute Angle – an angle whose measure is less than 90°. Y Z ǹ͓͔͒ is an acute angle.

C Obtuse Angle – an angle whose measure is more than 90° and less than 180°. D E ǹ̽̾̿ is an obtuse angle.

Two angles are complementary if the sum of their 25° 65° measures is 90°. A B ͡ǹ̻ ƍ ͡ǹ̼ Ɣ 25 ° ƍ 65 ° Ɣ 90 ° ǹ̻ and ǹ̼ are complementary angles.

Two angles are supplementary if the sum of their 40° 140° measures is 180°. C D ͡ǹ̽ ƍ ͡ǹ̾ Ɣ 40 ° ƍ 140 ° Ɣ 180 ° ǹ̽ and ǹ̾ are supplementary angles. D C Intersecting Lines – two lines that cross. A B E ž̻̽ʌʌʌʌľ intersects ̾̿žʌʌʌʌľ at point ̼. E F Parallel Lines – two lines in the same plane that do not intersect. G H ̿̀žʌʌʌʌľ Ǿ ́͂žʌʌʌʌľ is read “ ̿̀žʌʌʌʌľ is parallel to ́͂žʌʌʌʌľ.”

R

S Perpendicular Lines – two lines that intersect to P Q form right angles. T

͎͌žʌʌʌʌľ ɻ ž͊͋ʌʌʌʌľ is read “ ͎͌žʌʌʌʌľ is perpendicular to ž͊͋ʌʌʌʌľ.” ǹ͍͌͊ , ǹ͍͌͋ , ǹ͍͎͊ , and ǹ͍͎͋ are all right angles.

B 120° D A Vertical Angles – two angles with equal measure 60° 60° formed by two intersecting lines. E 120° C ǹ̼̻̿ and ǹ̻̾̽ are vertical angles. ǹ̼̻̾ and ǹ̻̿̽ are vertical angles.