I. Overview of knowledge points Today, with the rapid development of modern science and technology, with the popularization of computers, people are increasingly feeling its great superiority in daily work, study and life. However, it is also found that when solving some regular calculation problems, if the human brain can skillfully use calculation skills, it can get the calculation results faster than the computer. Students must hope that they can calculate quickly and correctly. So how do we do this? First of all, we should master the nature and law of operation skillfully; Secondly, we should pay attention to the characteristics of the topic and choose a reasonable and flexible calculation method. Second, induction and explanation of key knowledge. This lecture mainly introduces the common calculation skills of decimal multiplication and division to students. 1. By the method of decomposition, a number is properly decomposed into n numbers, rounded by using the commutative law, associative law and distributive law of multiplication, and a simple calculation is made. 2. Use the nature of multiplication and division to change the operation order and method. (1) The quotient of one number divided by another number and then divided by the third number is equal to the product of the first number divided by two or three; It is also equal to the quotient of the first number divided by the third number and then divided by the second number. That is, abc=acb=bca (2) divided by the third number is equal to the third number divided by any multiplier and then multiplied by another multiplier. 3. Use the property that the quotient is invariant: the dividend and divisor are multiplied (or divided) by the same number (except zero) at the same time, and the quotient is invariant. 4. Apply the property of product invariance: one factor is expanded several times, and the other factor is reduced by the same multiple at the same time, and the product is unchanged. Below, we will analyze and answer specific questions. Third, difficult knowledge analysis. Example 1, calculation:17.4837-174.8 1.9+17.4882 Analysis: move the decimal point of174.8 to the left, and/kloc. Then the distribution law of multiplication is used to simplify the calculation. Solution: 1 7.4837-174.5438+0.9+17.4882 =17.4837-17.4438+09+10. 3. 59. 9+6.5 10. 1 Analysis: Use the idea of integer rounding up, that is, make the number to be processed into whole ten, whole hundred, etc. , easy to calculate. Solution:13.59.9+6.510.1=13.5 (10.1)+6.5 (/kloc-0). 3.5 0.65433 solution:172.46.2+27240.38 =172.46.2+(1724+1000). 0.38 = 172.46 . 2+ 17240.38+ 10000.38 = 172.46 . 2+ 172.43 . 8+380 = 172.4(6.2+3.8)+380 = 1 72.465438+ solution: 5.2513.125485.2 = 5.25 (13.1254) 85.2 = 5.2552.585.2 = 0. Expand the dividend and divisor by 10 10 10 times at the same time, and divide them into integers, and then reduce the dividend and divisor by several times at the same time for simple calculation. You can also use the nature of division to change the operation order and method for simple calculation. Solution 1: (4.87.58.1) (2.42.52.7) = (487581) (242527) = (12425381) (66 = (1281) (69) = (2699) (69) = 29 =18 solution 2: (4.87.58.1) (2.42.52.7 =233 = 18 cases 6. Clever calculation: (702-2 13-4 14)3 analysis: divide the sum (difference) of two numbers by one number, and you can separate the two numbers (if divisible), and then find the sum (difference) of two quotients. Solution: (702-213-414) 3 = 7023-2133-4143 = 234-71-133.