Recognize a line of symmetry for a two-dimensional figure as a line across the figure such that the figure can be folded along the line into matching parts. Identify line-symmetric figures and draw lines of symmetry.
Recognize bilateral symmetry in nature. Recognize lines of symmetry by folding paper and using mirrors. Identify bilateral symmetry and draw the line of symmetry. Next, give your students this challenge. After that, this one. When providing the solution, give both the solution found in the video, and my much more elegant solution, which is as follows: Draw the lines of symmetry, a horizontal line and a vertical line. Consider one quadrant, whichever one you like. There are 4 triangles that occur entirely in that quadrant. These 4 triangles will occur 4 times, because there are 4 quadrants. There are 2 triangles which are cut by the vertical line of symmetry, but not the horizontal. These 2 triangles will occur 2 times, because there are 2 halves. There are 6 triangles that are cut by both lines of symmetry, and thus occur only once. Hence, the total number of triangles is$$4 \cdot 4 + 2 \cdot 2 + 6 = 26$$