1. addition operation:
(1) Use the carry idea: When the sum of single digits of two numbers exceeds 10, you can put the decimal digits in the tenth place and add the single digits directly. For example: 47+38, you can get 15 from 7+8 first, and put the rounded 5 in the tenth place. The unit number is 2, so the result is 85.
(2) Using approximation method: when one of the two numbers is close to the integer multiple of 10, the other number can be decomposed into several easy-to-calculate parts for operation and finally added. For example: 67+9, you can divide 9 by 7+2, then 67+7=74, and add 2 to get the result 76.
2. Subtraction operation:
Using the idea of borrowing: when the unit of the minuend is smaller than the unit of the minuend, you can borrow from the ten digits and subtract the number after borrowing. For example, 63-29, where 3 digits are less than 9, can be borrowed from 10 digits to 53- 19, and then the digits are 3 and 5, so the result is 34.
3. Multiplication operation:
(1) Use the 99 multiplication table: memorize the 99 multiplication table and find the corresponding results as needed. For example: 8 times 6, you can find the intersection of 8 rows and 6 columns in the multiplication table of 99, which is 48.
(2) Using the multiple relation: When a number is a multiple of 10, the multiplication operation can be simplified as adding the number of another number with several corresponding zeros. For example: 40 times 7, you can add a 0 after 7 to get 280.
4. Further expand:
(1) Fast estimation: In oral calculation, the logarithm can be simply estimated to get a faster result. For example: 87+65, 87 can be estimated as 90, 65 as 70, and then 90+70= 160, and then fine-tune according to the estimated error to get accurate results.
(2) Decomposition and combination of numbers: In the process of operation, the numbers can be decomposed and combined to make the calculation easier. For example: 57+48, you can decompose 57 into 50+7, 48 into 40+8, then 50+40=90, 7+8= 15, and finally see 90+ 15= 105, and then adjust it according to the carry.