# Chapter 5 Plane Geometry

## 42 terms

names a location

### line

perfectly straight and extends forever in both directions

### plane

perfectly flat surface that extends forever in all directions

### segment

the part of a line between two points

### ray

part of a line that starts at one point and extends forever in one direction

### angle

formed by two rays with a common endpoint called the vertex

measures 90º

### acute angle

measures less than 90º

### obtuse angle

measures greater than 90º

### parallel lines

two lines in a plane that never meet

### perpendicular lines

two lines that intersect at 90º angles

### transversal

a line that intersects any two or more other lines

### Properties of Transversals to Parallel lines

the acute angles that are formed are all congruent; the obtuse angles are all congruent; any acute angle is supplementary to any obtuse angle.

### Triangle Sum Theorem

The angle measures of a triangle in a plane add to 180 degrees

3 acute angles

1 right angle

### equilateral triangle

3 congruent sides and 3 congruent angles

### isosceles triangle

at least 2 congruent sides and 2 congruent angles

### scalene triangle

no congruent sides and no congruent angles

### polygon

a closed plane figure formed by three or more segments

### regular polygon

a polygon where all sides and angles have equal measure

### trapezoid

exactly 1 pair of parallel sides

### parallelogram

has 2 pairs of parallel sides

### rectangle

has 4 right angles

### rhombus

has 4 congruent sides

### square

4 congruent sides and 4 right angles

### slope

a number that describes how steep a line is

vertical change

### run

horizontal change

### correspondence

a way of matching up two sets of objects

### transformation

translations, rotations, reflections

### translation

slides a figure along a line without turning

### rotation

turns the figure around a point called the center of rotation

### reflection

flips the figure across a line to create a mirror image

### line symmetry

a line through an image so that the two sides are mirror images

### Line of Symmetry

the name of the line used in line symmetry

### tessellation

a repeating pattern of plane figures that completely covers a plane with no gaps or overlaps

### regular tessellation

a regular polygon is repeated to fill a plane. The angles at each vertex add to 360º

### semiregular tessellation

two or more regular polygons are repeated to fill the plane and the vertices are all identical.

### Obtuse triangle

Has one obtuse angle