What is the formula of one-to-one multiplication?

The formula for primary school students to learn "primary ninety-nine" is from "11" to "998 1", but in ancient times it was the other way around, from "998 1" to "224". Because the first two words of the formula are "99", people call it "Xiao 99" for short. It was only in the 13 and 14 centuries that this was reversed: "One gets one ... 998 1".

Features of Jiujiu Watch:

General 1 and 99 tables only use 9 numbers from 1 to 9.

2. Jiujiu table contains the commutativity of multiplication, so only 8972 is needed, and "9872" is not needed. 9 times 9 has 8 1 product, and 99 table only needs 45 product. In the Ming Dynasty, abacus also had 99 tables with 8 1 product. The 99 table of 45 items is called small 99, and the 99 table of 8 1 item is called big 99.

3. The shortest multiplication table in the ancient world. Maya multiplication table must have 190, Babylon multiplication table must have 1770, and Egypt, Greece, Rome, India and other countries must have infinite multiplication tables; Only 45/8 1 item is needed for the 99 table.

4. There is rhythm when reading aloud, which is convenient for memorizing the whole table.

5. Jiujiu watches have existed for at least 3000 years. It was used for calculation from the Spring and Autumn Period and the Warring States Period, and was improved and used for abacus in the Ming Dynasty. The 99 standard is also the basic skill of elementary school arithmetic.