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The A,B,C’s of Geometry – PART 1 4 th grade


The Basics




This is Ray AB . You can also call him Ray A . Write his name as Ray AB or just AB for short . endpoint Ray There is only ONE endpoint on a RAY , and the endpoint is ALWAYS named or written first .


Line Segment


C D Line Segment


C D NAMES : Line Segment CD or Line Segment DC WRITTEN : CD or DC endpoint endpoint Line Segment There are two endpoints on a LINE SEGMENT . It doesn’t matter which endpoint is named or written first.


C D NAMES : Line CD or Line DC WRITTEN : CD or DC No endpoint! No endpoint! There are no endpoints on a LINE . It doesn’t matter which point is named or written first. Line


Lines,Line Segments, Rays t hey cross t hey never cross t hey cross a t 90°


s pelled ang le ….not ang el ! The sides of angles are made of rays or line segments Angle 2 rays with a common endpoint. 2 line segments with a common endpoint.


Name an angle by 3 points. The middle point is called the vertex . Or, name it by the vertex on its own. So, the name of this angle is: Angle XYZ , Angle ZYX, or just Angle Y Angle X Y Z


Written as: Angle XYZ, Angle ZYX Angle Y or or Angle X Y Z


small 3 Kinds of Angles medium large a cute angle obtuse angle > 90° < 90° = 90° 0° 90° 180° Angles are measured in degrees . The symbol for degrees is ° right angle Angles “open up” from 0°


Right Angles 0° 180° m easure exactly 90° 90°


Acute Angles 0° 90° 180° m easure between 0° and 90° 45° is halfway b etween 0° and 90°


Obtuse Angles 0° 90° 180° m easure between 90° and 180° 135° is halfway b etween 90° and 180°


0° 90° 180° 270° 360 ° “full circle” = All Angles Halfway around = 180° All the way around = 360°


4 Kinds ° Triangles Every triangle has 3 angles. All 3 angles add up to 180° 60° 60° 60° 9 0° different° same°


One Picture that Puts it All Together Triangles >90° 2 sides equal all sides equal < 90° n o sides equal >90° 2 sides equal < 90° n o sides equal < 90° =90° 2 sides equal n o sides equal =90° √ √


Quadrilaterals 4-sided shapes Trapezoid Rectangle Rhombus Parallelogram Square


= 3 =4 =4 = 5 4 Equal Sides Opposite Sides are Parallel Opposite Sides are Equal Opposite Angles are Equal 4 right Angles This chart shows why a SQUARE is also a parallelogram, a rectangle, and a rhombus, and why they are NOT a square! Compare t hese 4


A Rhombus looks like a Square that got pushed. A Parallelogram looks like a Rectangle that got pushed. EASY to Remember:


Trapezoids What’s different a bout them? Trapezoids have only ONE pair of parallel sides, instead of two. These other Quadrilaterals have TWO pairs of parallel sides: This pair of sides isn’t p arallel.


Symmetry A shape has symmetry if it can be folded in half along a straight line AND the two halves completely overlap, or cover, each other. Some shapes have no have symmetry: Scalene triangle – 0 lines of symmetry Rectangle, star, equilateral triangle – many lines of symmetry Isosceles triangle, arrow, butterfly – = 1 line of symmetry Some shapes have more than 1 line of symmetry: ❶ ❷ ❸


End of Part 1

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