Lesson 4

The purpose of this lesson is to introduce the concept of line symmetry and for students to identify lines of symmetry in two-dimensional figures.

This lesson introduces students to symmetry as an attribute of two-dimensional figures. Students relate lines of symmetry to the lines of folding that create two identical halves: if a figure is folded along or mirrored across such a line, the two halves would overlap and match exactly. The idea of folding along or mirroring across a line is essential here, as there are some figures that could be decomposed into two identical halves but have no line symmetry.

After reasoning about the meaning of lines of symmetry from examples and non-examples, students work to identify figures that have line symmetry and draw lines of symmetry.

• Make copies of the set of figures in the second question available for cutting and for demonstration during the lesson synthesis.

• Sort the shape cards from the previous lessons into three groups of 12 cards (A–L, M–X, and Y–JJ).

What ideas do students have about symmetry? How will you leverage those ideas in the next lesson?