Arabic numerals were invented by ancient Indians and later spread to Arabia and Europe. Europeans mistakenly thought it was invented by Arabs, so they called it "Arabic numerals". Because it has been circulated for many years, people call it Yi, so people still mistakenly believe that these numerical symbols invented by ancient Indians are called Arabic numerals.

multiplication table

Jiujiuge is the multiplication formula we use now. As early as the Spring and Autumn Period and the Warring States Period BC, Jiujiu songs have been widely used by people. In many works at that time, there were records about Jiujiu songs. The original 99 songs started from "99.8 1" to "22.24", with 36 sentences. Because it started with "998 1", it was named 99 Song.

The expansion of Jiujiu Song to "One for One" was between the 5th century and10th century. It was in the 13 and 14 centuries that the order of Jiujiu songs became the same as it is now, from "one for one" to "9981". At present, there are two kinds of multiplication formulas used in China. One is a 45-sentence formula, usually called "Xiao Jiujiu"; There is also a sentence 8 1, which is usually called "Big Uncle Nine".

Third, Mobius ring

Mobius ring is a topological structure with only one face and one boundary. You can twist a piece of paper into 180 degrees, and then stick the two ends together to form a Mobius ring.

Interesting mathematical knowledge:

1. If the length of "one foot" is 8 cm and the length of the desk is 7 feet, then the length of the desk is 56 cm. If each step is 65 cm, then when you go to school, you can count how many steps you have taken and calculate how far it is from home to school.

2. Height is also a ruler. If the height is 150 cm, hold a big tree and put your hands together. The length of this tree is about 150 cm. Because everyone's arms are flat, the length and height between the fingertips of both hands are similar.

3. If you want to measure the height of the tree, shadows are also helpful. Just measure the length of the shadow of the tree and your own shadow. Because the height of the tree = shadow length × height ÷ graph length.

If you want to know how far the mountain ahead is from you, you can ask the sound to help you measure it. Sound can travel 33 1 meter per second, so you can hear the echo by shouting at the mountain and watching it for a few seconds. Multiply the time to hear the echo by 33 1 and divide it by 2.

5、? "Astronomical Recorder" Coral scientists found that corals record time on themselves: they "carve" an annual ring pattern on the body wall every day and "carve" 365 annual rings a year, no more or less.

So, if you want to know their age, just count the annual rings on their body walls. Scientists also found that 350 million years ago, corals "carved" 400 rings instead of 365. The reason is that the earth's rotation was only 2 1.9 hours a day, not 365 days a year, but 400 days.