Multiply by 5 quickly.
Multiply a number by 5, multiply it by 10 and divide it by 2. For example, 5 × 7 = (10 × 7) ÷ 2 = 35.
Multiply by 9 quickly
Multiply a number by 9, then multiply it by 10, and then subtract the number itself. For example, 9 × 7 = (10 × 7)-7 = 63.
Quickly multiply by 1 1.
Multiply a two-digit number by 1 1, just add up its digits and insert this sum into the middle of the original number. For example, 1 1 × 34 = 374.
Fast square
To square a number, you only need to square the single digits of the number, and then double the product of the ten digits and the single digits. Like 42? = 16 + 2 × 4 × 10 + 40? = 1764。
Divide by 2 quickly.
Divide an even number by 2, just shift it to the right by one bit. For example, 46 ÷ 2 = 23.
Divide by 4 quickly.
To divide a number by 4, just move it two places to the right. For example, 256 ÷ 4 = 64.
Divide by 5 quickly.
Divide a number by 5, multiply it by 2 and then divide it by 10. For example, 245 ÷ 5 = (245 × 2) ÷ 10 = 49.
Divide by 9 quickly
Divide a number by 9, just add up its digits, and then repeat this operation until you get a digit. For example, 567 ÷ 9 = 6.
Quickly divide by 1 1.
Divide a two-digit number by 1 1, just subtract each number and insert the difference into the middle of the original number. For example, 462 ÷ 1 1 = 42.
In a word, fast math skills can help us to calculate math problems quickly and improve the efficiency of calculation. These skills need to be practiced repeatedly and mastered skillfully before they can play a role in practical application.