Definition 1: Two geometric figures are similar if the are the same shape but different sizes. This means that if you take a picture of the second image and shrink (zoomout) or expand (zoomin) it, the figures are congruent.The symbol for similarity is \sim . For example, {\triangle \text{ABC}}\, \sim \,\triangle \text{DEF} is read "triangles АВС and DEF are similar". Example:

Similarity is a weaker condition than congruency (which requires both same shape and size). Mathematically, two figures are similar if the corresponding angles are the same size and the corresponding sides are equally proportioned. Related topics:
