Because each number can only be used once, and it is subtraction.

The problem of discovering mathematical laws refers to a problem that has nothing to do with the mathematical laws that students have learned before. Students are required to find out the laws from known things before they can solve them. Most of the math problems that students do belong to the first category.

Subtraction follows several important patterns.

It is anti-commutative, which means that changing the order will change the sign of the answer. It is not associative law, that is, when a subtraction exceeds two numbers, the order of subtraction is very important.

Subtracting 0 does not change a number. Subtraction also follows predictable rules related to addition and multiplication. These laws can be proved, starting with the subtraction of integers and summarizing them with real numbers and other things. Continue the general binary operation of these patterns and learn in abstract algebra.