In mathematics, proportion is the proportion of the number of parts in a population to the total, which is used to reflect the composition or structure of a population. Two related quantities, one of which changes and the other changes with it. Judging whether two ratios can form a proportion depends on whether their ratios are equal. The nature of proportion: the basic nature of proportion: the four numbers that make up the proportion are called proportional terms. The two terms at both ends are called external terms of proportion, and the two terms in the middle are called internal terms of proportion. In proportion, the product of two external terms is equal to the product of two internal terms. It is a common proportional property in algebra, which mainly includes the combined ratio property, the divided ratio property, the combined ratio property, the equal ratio property and its generalization. These four properties are mostly used in the calculation and proof of fractions, and the application of trigonometric functions, similar triangles and the proportionality theorem of parallel lines. Among them, the equal ratio property is the most widely used.