Week 8 · Fact families and quadrilaterals · 3 lessons
2
Sorting flat shapes
Discover Mathematics P3: Term 1, Lessons 21–30
GoalSort flat shapes by their sides, corners and equal sides, and name the rule that was used.
New words
attribute · a feature of a shape, such as its number of sidespolygon · a closed flat shape with straight sidesquadrilateral · a flat shape with 4 straight sidesright angle · a square corner, like the corner of a page
Explanation
We can sort shapes by their attributes. Count the sides and the corners. Look for sides of equal length and for square corners, which are called right angles. A polygon is a closed shape with straight sides, so a circle is not a polygon. A triangle has 3 sides, a quadrilateral has 4, a pentagon has 5, a hexagon has 6 and an octagon has 8. The square, the rectangle, the rhombus and the trapezium are all quadrilaterals. One shape can fit more than one rule. A square fits the rule 4 sides and also the rule 4 equal sides.
Worked examples
Rule: the shape has 4 sides. Check each one. A is a triangle (3 sides), B is a square (4), C is a pentagon (5) and D is a trapezium (4). B and D fit the rule.
Shape
Sides
Square corners
Equal sides
Square
4
4
4
Rectangle
4
4
2 pairs
Rhombus
4
0
4
This table sorts three quadrilaterals by their attributes. The square and the rectangle both have 4 square corners. The square and the rhombus both have 4 equal sides. The rhombus has no square corners.
Mystery shape: I have 8 straight sides and 8 corners. Which shape am I? Count the sides of each: A has 5, B has 8 and C has 6. I am shape B, the octagon.
Class activity
Shape sort. Cut 10 shapes from paper: 2 triangles, 2 squares, 2 rectangles, 1 pentagon, 1 hexagon, 1 circle and 1 long diamond. Sort them by the number of sides. Then sort them again by square corners. Which shapes stay together both times?
Practice
1
Which shapes are quadrilaterals?
B and D, the square and the rectangle. They have 4 sides.
2
Which shape is not a polygon? Why?
The circle, A. It has no straight sides.
3
How many sides and how many corners does an octagon have?
8 sides and 8 corners.
4
Which shape has 4 equal sides and no square corners?
