Expression: (a+b) (a-b) = a 2-b 2, and the product of the sum of two numbers and the difference of two numbers is equal to the square difference of two numbers. This formula is called the square difference formula of multiplication.

When the division formula is the sum of two numbers and the difference between the two numbers is multiplied, the product is binomial. This is because when two binomials with such characteristics are multiplied, two of the four terms of the product will be opposite. The result of the merger of these two items is zero, so there are only two items left. And their product is equal to the square difference of these two numbers in the multiplication formula, that is, (a+b) (a-b) = a 2-b 2, and the product of the sum of two numbers and the difference of these two numbers is their square difference.

[Inverse derivation of square difference formula]

=a^2-b^2+(ab-ab)=(a^2-ab)+(ab-b^2)

=a(a-b)+b(a-b)

=(a+b)(a-b)