What is a definition?• Known words used to describe a new wordUndefined terms – not formally defined There are 3 in geometry. Point Line Plane
Point1. Has no dimension (no length, width, thickness) 2. Usually represented by a dot 3. Named using one capital letter • B
Line1. Extends forever in one dimension (length)2. Has an arrowhead on each end representing the fact that it goes on forever3. Consists of an infinite amount of points4. Always straight5. Named with a lowercase script letter or by 2 points on the line Can be called: l Line l A Line AB or AB B Line BA or BA
Plane1. Extends forever in 2 dimensions (length & width)2. A flat surface consisting of infinitely many points3. Usually represented by a 4-sided figure4. Named with a capital script letter or 3 points on the surface of the plane A W Can be named: C Plane W B Plane ABC, plane BCA, plane CBA, etc.
More Definitions• Collinear points – points that lie on the same line• Coplanar points – points that lie on the same plane Turn to page 10 in your book; look at example #1.
Line Segment (or segment)• A section of a line that has 2 endpoints A B• Named by its endpoints. AB or BA
Ray• Piece of a line with only one endpoint (initial point) and continues forever in the other direction B A• Named by the endpoint and a second point named on the ray. (name MUST begin with the endpoint!) AB
Opposite Rays• 2 rays that share a common initial point and face opposite directions. P Q S• QP and QS are opposite rays.
Example: 6 volunteers please!• Draw & label points A & B.• Draw & label points C & D.• Draw AB.• Draw BC.• Draw AC.• Draw DA.
More Definitions• Intersect – 2 or more figures intersect if they have one or more points in common.• Intersection – all points or sets of points the figures have in common• What is the intersection of: AB & DA BC & AC BC & BC
When 2 planes intersect,their intersection is a line.