Represents the "total amount of substances" of two or more substances. Relational expression refers to expressing the quantitative relationship of variables with mathematical symbols and words, reflecting the internal relationship of mathematical objects, and is one of the important tools for mathematical research and problem solving.

Relationships include equality, inequality and functional relationships. Y is equal to 2x plus 1, that is, the functional relationship expressed by mathematical symbols, that is to say, X has a function, and the value of Y is twice of X plus 1.