Figures are mathematically similar when their corresponding angles are congruent and when the ratios of the lengths of their corresponding sides are equal. This is related to finding a scale factor. Figures are also mathematically similar when the ratios of side lengths within the figure are equivalent.
Any two rectangles are similar.
FALSE
This is false because any two rectangles could have different ratios of the lengths of their corresponding sides.
Any two equilateral triangles are similar
TRUE
Since they always have three angles that are each 60 degrees, any two equilateral triangles are similar. And the sum of all of the angles is 180 degrees. So this shows that all of the corresponding angles of two equilateral triangles will always be congruent.