Applicable age: 7-9 years old, second and third grades
Hello friends, we met again, in the previous video, the teacher explained how to use mental arithmetic to quickly calculate the addition of two and three digits, so today we summarize the idea of mental arithmetic, so that everyone can more clearly and fully understand the idea of mental arithmetic and the mathematical thinking contained in it. Start with mental arithmetic in elementary schools: Change your school calculation habits, from left to right is easier!
1. From left to right
In school, we will learn the basic vertical calculation in the second grade, as we all know, the calculation law of vertical calculation is from the individual bit to the highest position, which is a calculation method that starts from the low position and goes from right to left, which is the most basic law of vertical calculation. Vertical calculations are used in pen calculations, but in mental arithmetic, we cannot calculate from right to left.
First of all, there are 2 points, from right to left calculation, we can only start from the smallest number of digits, can not very quickly estimate the approximate range of numbers, does not meet the requirements of quick answers. Second, right-to-left calculations are more suitable for written calculations, which will be more accurate but the process will be more cumbersome. In addition to our mental arithmetic can quickly get the answer, we should also quickly estimate the scope of the answer, so the secret of mental arithmetic is that it needs to be calculated from left to right, that is, it is because of the need for reverse calculation, here the teacher should remind everyone that if you have not fully mastered vertical calculation, it is not recommended that you not rush to learn mental arithmetic, because our mental arithmetic foundation is actually based on the logic of vertical calculation that everyone fully understands.
2. Double-digit addition
Now that we all know that mental arithmetic is calculated left-to-right, that is, it is calculated from a high position, so let's start with a simple calculation. First of all, I will give an example, calculate 45 + 87, if we do the direct calculation, that is, 5 and 7 add 2 into 1, 8 and 4 add up, plus the 1 of the carry, add up to get 13, that is 132. But if we directly use mental arithmetic, that is, to start from the high level, the answer can be obtained very quickly, first the large number of digits added up 45 + 80 = 125, followed by 125 + 7 = 132, which is equivalent to the first time only need to calculate 4 + 8, the second time only need to calculate 5 + 7 can be.
Let's take another example of 36+77, in the same way, we first use 36 plus the highest bit (ten bits) of 7, that is, 36 + 70 = 106, and then calculate 106 + 7 = 113. This way of calculation requires everyone to practice more and master the core of calculation, so that we can calculate faster than other children in the exam, and everyone will practice the following questions.
3. Three-digit addition
Three-digit addition and two-digit addition are calculated in the same way, if you are very skilled in adding two-digit addition, then the addition of three digits will become very fast, and you will feel that the addition calculation is actually very simple. For example, 538 + 327, this question we first use 538 plus 300, then add 20, and finally add 7, that is, 538 + 327 (300 + 20 + 7) = 538 + 300 + 20 + 7 = 838 + 20 + 7 = 858 + 7 = 865.
In fact, all additions can be calculated in this way, with the aim of converting a slightly more complex mathematical problem into a single-digit addition. In our minds, we only need to remember these two three digits, that is, 6 numbers, and then add the hundred and the hundred, add the ten and ten, add the number and the number, and add up one by one, just say that we have to start from the large number of digits, so that we can get the answer quickly. Let's calculate the following problem 623 +159, the same first use 623 + 100 = 723, then use 723 + 50 = 773, and finally use 773 + 9 = 782. In order to achieve proficiency in the use of this method of mental arithmetic, we must fully practice the following topics.
With the above two examples, do you think that the left-to-right algorithm can greatly reduce the difficulty of the problem and simplify the problem? This kind of calculation, in addition to calculating 2 three-digit addition or 2 two-digit numbers, we can actually apply this algorithm to three digits plus two digits or a larger number of digits, as long as it is an addition operation, you can try to calculate in this way, but everyone must use more in life to really master this mental arithmetic method.
The above is the teacher for everyone to explain the mental arithmetic method from left to right, today's explanation is the addition of two digits and three digits, we must practice diligently, the teacher gave some more complex practice topics below, we must try to use this calculation method to complete yo.
If you have not mastered this method, you can pay attention to the teacher and watch the teacher's previous video. We'll see you next time.