A negative number is a positive number, which means that two negative numbers are multiplied to finally get a positive number. The multiplication law "negative is positive" is just a stipulation, and the arithmetic of numbers is stipulated, not deduced. Specify arithmetic first, and then study whether the law of arithmetic is established.

Linguistically speaking, double negation equals affirmation; Mathematically, this is similar to reciprocal. The way to find the reciprocal of a negative number is to add "-"in front of it, and then the reciprocal of a negative number is positive, that is, negative is positive. (-1)×(- 1) is equal to 1 or-1; It is proved that (-1) × 1 =- 1, so (-1) × (-1) =1.

In mathematical multiplication, the reason why negative is positive is explained as follows:

1. M Klein, an American mathematical historian and educator, solved the problem of "two negative numbers multiplied by positive" through the debt model: a person owes 5 yuan every day, and he owes 15 yuan (0 yuan) three days after a given date. If 5 yuan Qian's house is marked as -5, then "5 yuan Qian is in debt for three days every day" can be mathematically expressed as: 3×(-5)=- 15.

The same person owes money to 5 yuan every day, so three days before the given date (0 yuan), his property is more than the given date 15 yuan. If we use -3 for three days ago and -5 for daily liabilities, then his economic situation three days ago is expressed as (-3)×(-5)= 15.

2. Reciprocal model

5× 3 = 5+5+5 =15, (-5 )× 3 = (-5)+(-5) =-15. Therefore, if a factor is replaced by its inverse, the product obtained is the inverse of the original product. Therefore,