complex numbers, . When , these similarities are isometries. Area ratio and volume ratio The ratio between the areas of similar figures is equal to the square of the ratio of corresponding lengths of those figures (for example, when the side of a square or the radius of a circle is multiplied by three, its area is multiplied by nine — i.e. by three squared). The altitudes of similar triangles are in the same ratio as corresponding sides. If a triangle has a side of length and an altitude drawn to that side of length then a similar triangle with corresponding side of length will have an altitude drawn to that side of length . The area of the first triangle is, , while the area of the similar triangle will be . Similar figures which can be decomposed into similar triangles will have areas related in the same way. The relationship holds for figures that are not rectifiable as well. The ratio between the volumes of similar figures is equal to the cube of the ratio of corresponding lengths of those figures (for example, when the edge of a cube or the radius of a sphere is multiplied by three, its volume is multiplied by 27 — i.e. by three cubed). Galileo's square–cube law concerns similar solids. If the ratio of similitude (ratio of corresponding sides) between the solids is , then the ratio of surface areas of the solids will be , while the ratio of volumes will be . Similarity with a center If a similarity has exactly one invariant point: a point that the similarity keeps unchanged, then this only point is called "center" of the similarity. On the first image below the title, on the left, one or another similarity shrinks a regular polygon into a concentric one, the vertices of which are each on a side of the previous polygon. This rotational reduction is repeated, so the initial polygon is extended into an abyss of regular polygons. The center of the similarity is the common center of the successive polygons. A red segment joins a vertex of the initial polygon to its image under the similarity, followed by a red segment going to the following image of vertex, and so on to form a spiral. Actually we can see more than three direct similarities on this first image, because every regular polygon is invariant