What is the difference between quick calculation formula and traditional vertical calculation?

The traditional calculation formula is gradual and suitable for everyone at any time, while the fast calculation formula is based on the traditional calculation, and then special provisions are made, such as 25× 25 = 625. To calculate vertically, multiply five of two digits to get 25, and then calculate two of ten digits. Before multiplying the two, add 1 to one of them and then multiply the other two to get 6, which is 625. Second, the ten digits of the two digits are the same. The single digits of three or two numbers must be able to synthesize a number of 10. 15 and 15, 25 and 25, 14 and 16, 23 and 27, 52 and 58, 66 and 64, and their multiplication can be calculated as follows. By analogy, there are many rules or laws for multiplying the numbers in this interval with those in that interval, so it is necessary to remember them well to be handy. So this fast algorithm may not be useful for ordinary people. More suitable for people who often count.