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Definitions:  2D shapes

Page history last edited by Sandra McVannel 9 years, 3 months ago




Definitions:  2D shapes


This page will provide definitions for the vocabulary we will be using during the two-dimensional geometry unit.    Definitions come from various sources, of which, the easiest definition was chosen for students to understand.



polygon:  a closed figure with three or more straight sides.  We name a polygon by the number of its sides.  (ie. a five-sided polygon is a pentagon)



Number of sides

on the polygon                 Name of the Polygon


*3  triangle 

square, rectangle

rhombus, trapezoid 

*5  pentagon 
*6  hexagon 
*8  octagon 
10  decagon 
11 hendecagon
12 dodecagon
13 tridecagon
20 icosagon

*grade three students only need to know the * names




Angle:  two sides of a figure, or lines, meet to form an angle


Right angle:  two sides of a figure, or lines, that make a square corner at the vertex form a right angle.  


Obtuse angle:  two sides of a figure, or lines that make an angle larger than a right angle.


Acute angle:  two sides of a figure, or lines that make an angle smaller than a right angle.



Congruent:  figures (shapes) that have the same size and same shape.



Figure:  a geometic drawing or diagram


Parallel lines: two lines that are always the same distance apart and never meet




Parallelogram:  a figure with 4 sides where 2 pairs of opposite sides are parallel

                      *we use arrows to show which pairs of lines

                                              are parallel


Regular figure:  has all sides equal and all angles are equal.  A square is a regular figure.


Rhombus:  a parallelogram with 4 equal sides.



Reflection (Flip):  reflects a figure in a line to create a congruent image.



Rotation (Turn):  a turn moves a figure around a turn centre.  After 1 turn, the figure is back to where it started.


Slide (Translation):  a slide moves a figure along a line.  The figure does not turn.  



Trapezoid:  a figure with 4 sides where 2 sides are parallel.





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