# How Many Lines of Symmetry Does a Rhombus Have?

A rhombus is a four-sided figure in which all sides are equal in length. It is also known as a diamond, since it looks like a stretched-out version of the diamond shape. Rhombuses are interesting shapes to study because they have several unique properties. One of the properties of rhombuses that we will focus on in this article is their line(s) of symmetry.

A line of symmetry is a line that divides a shape into two exact mirror images. In other words, if you were to fold the shape along this line, both halves would match up perfectly. Some shapes have multiple lines of symmetry, while others may have only one or none at all. So, how many lines of symmetry does a rhombus have?

To answer this question, we first need to understand what makes a line of symmetry. For a line to be a line of symmetry for a rhombus, it must meet two criteria. First, the line must pass through the midpoint of two opposite sides of the rhombus. Second, the line must bisect the angles formed by those sides.

Let’s take a look at an example to better understand these criteria. Imagine you have a rhombus with sides of length 6 units. Draw a line from the midpoint of the left side to the midpoint of the right side, and extend it out until it intersects the top and bottom sides of the rhombus. This line meets the first criteria, since it passes through the midpoint of opposite sides.

Now, let’s check if this line meets the second criteria by measuring the angles it forms. To do this, draw perpendicular lines from the endpoints of the line we just drew to the opposite sides of the rhombus. These lines create four smaller triangles within the rhombus. Since our rhombus has four equal sides, we know that each of these triangles is congruent. Therefore, their angles are also congruent.

We can calculate the angles of these triangles by using the Pythagorean theorem to find the length of their height (which is half the length of a side of the rhombus) and the length of their base (which is the distance between the midpoints of opposite sides).

Using our example of a rhombus with sides of length 6 units, we know that the height of each triangle is:

$$\sqrt{6^2 – 3^2} = \sqrt{27}$$

And the distance between the midpoints of opposite sides (which is also the length of the line we drew) is:

$$\sqrt{(6/2)^2 + (6/2)^2} = \sqrt{18}$$

Using trigonometry, we can find the angle formed by each triangle:

$$\tan^{-1}(\frac{\sqrt{27}}{\sqrt{18}}) \approx 56.31^\circ$$

Therefore, the angle formed by the line we drew is twice this amount, or approximately 112.62 degrees.

Now, let’s look at the other two angles formed by this line. We know that all four angles of a rhombus are congruent, so each of the two remaining angles must be (180 – 112.62)/2 = 33.69 degrees.

So, does the line we drew bisect these angles? Yes, it does! By measuring them, we see that each is made up of two 33.69 degree angles, which sum to 67.38 degrees. Since the angle formed by the line we drew is 112.62 degrees, and 112.62 + 67.38 = 180 degrees, we know that this line passes through the midpoint of the top and bottom sides and bisects the angles formed by those sides.

Therefore, this line is a line of symmetry for our rhombus. And since we can draw three more lines that meet the same criteria (one for each pair of opposite sides), we know that a rhombus has four lines of symmetry.

To summarize, a line of symmetry for a rhombus must pass through the midpoint of opposite sides and bisect the angles formed by those sides. By drawing lines that meet these criteria, we can see that a rhombus has four lines of symmetry. This property is unique to rhombuses and is just one of the many interesting characteristics of this fascinating shape.