The Three Meanings of Similarity

We can say that two objects are similar if they look alike. In layman’s words, objects with the same shape, whether they have the same size or not are usually called similar. In mathematics, it is quite different. In this post, we are going to learn the three mathematical meanings of similarity.

In mathematics, two objects are similar when either one of the following three conditions is true.

1. When one figure is reduced or enlarged, it will become congruent with the other

The first meaning is based on the definition of congruence. That is, when two figures are similar, if one figure is enlarged or reduced, then they will become congruent with the other. This definition is better illustrated graphically, using a drawing or an applet just as the one shown below.  » Read more

Lines, Planes, and Perspective

Aside from points, as we have discussed in the previous post, the other two undefined terms in Geometry are lines and planes. A line may be drawn through two points, while three points are needed to determine a plane. The representations of these undefined terms are the building blocks of Euclidean Geometry.  They can be combined to create shapes, drawings, and sketches such as the painting shown in the first figure. Looking at the painting makes us realize that almost all the things around us are  mostly basic geometric shapes.

In the painting above, we can easily see geometric shapes such as rectangles, triangles, trapezoids , and parallelograms. We can also see curves and arcs in vases, flowers, and fruits.  Notice that although the painting seems to be only made by these shapes, the artist has made it look very realistic. For example, the window frames located at the left side of the painting are of the same size, but the artist made the ‘nearer’ frame larger to give a somewhat three dimensional effect.  In doing so, the painter considered the distance of the window frames from the observer.  The farther the frame, the smaller its size. Observe that this technique is more apparent in the painting by Vincent Van Gogh in the second figure. » Read more

Understanding the Meaning of Correspondence

In Geometry, two objects are congruent if they have the same size and shape. Two triangles drawn on a piece of paper are congruent if we can cut them out with scissors, and superimpose them to fit exactly, that is, without gaps or overlaps. If the triangles fit exactly, the corresponding parts are the parts that coincide. Consequently, corresponding parts of congruent triangles are congruent. Therefore, if two triangles are congruent, then their corresponding angles are congruent and their corresponding sides are also congruent.

In the figure above, if we superimpose the two triangles, $\overline {AB}$ will coincide with $\overline {DE}$ and $\angle C$ will coincide with $\angle F$. Hence $\overline {AB}$ and $\overline {DE}$ are corresponding sides and $\angle C$ and $\angle F$ are corresponding angles. » Read more

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