Final answer:
The proportion X1X2 = Y1Y2 is an equation of similarity in mathematics.
Step-by-step explanation:
The proportion X1X2 = Y1Y2 represents an equation of similarity. In mathematics, similarity is a concept used to describe figures that have the same shape but may differ in size. When two sets of corresponding sides of two figures have the same ratio, they are considered similar. In this case, X1X2 and Y1Y2 have the same ratio, indicating that the two figures are similar.