109
Line symmetry
Textbook: Part 4, lesson 109, pp. 4–6
GoalLine symmetry: the axis of symmetry; vertical, horizontal and diagonal axes; counting the axes of shapes.
New words
symmetry · simmetriyaaxis of symmetry · simmetriya o‘qisymmetric shape · simmetrik shaklcoinciding · ustma-ust tushish
Explanation
If folding a shape along a line makes its two halves lie exactly on each other, that line is an axis of symmetry. An axis can be vertical, horizontal or diagonal. A square has 4 axes of symmetry (2 middle lines and 2 diagonals), a rectangle that is not a square has 2, an equilateral triangle has 3, and a circle has infinitely many. Some shapes have no axis at all.
Worked examples
The letters A, M and T have a vertical axis; B, C and D have a horizontal axis; H has both; F has none.
A butterfly's wings are symmetric: folding along the axis puts one wing on the other.
Class activity
“Fold to check”: children cut out a square, rectangle, triangle and circle from paper (an adult helps with scissors) and fold them to find the number of axes of symmetry.
Practice
1
How many axes of symmetry does a square have?
4
2
How many axes of symmetry does a rectangle that is not a square have?
2
3
How many axes of symmetry does an equilateral triangle have?
3
4
Which of the letters A, F, H, S have an axis of symmetry?
A, H
5
An axis of symmetry splits a square into two equal parts. What percent of the square is one part?
50