Yesterday, Tuan Tuan's math exam was not very satisfactory. Almost all mistakes are related to abdication subtraction within 20. I asked his teacher what method he wanted him to use, and he said that the teacher only asked to recite 20 as the calculation of the minuend. I don't know if it is. But the problem still needs to be solved. Teach him yourself first,18-9 =10+8-9 =10-9+8 = 9. But he is still easy to make mistakes and doesn't understand the exchange method. Let's ask the primary school teacher for help and find a teacher's teaching experience. It seems that the method I taught him is similar to the "breaking ten methods" in the post, but the teacher's description is much easier. I'm going to teach him after school today. The reposts are as follows (appropriate refinement): Teaching research on the addition and subtraction of numbers within 20; Several methods and counting methods of carry addition in 20: 1. Count them one by one by hand or by drawing dots. 2. Number method. It's basically a counting method, just counting one by one on the basis of the first number. For example, 8+5, the first number 8 is not counted, and then the number 8+1+1+kloc-0/+0+1. 3, two-handed memory method. Poor students can use the "two-handed memory method" to help. One hand represents a number, the thumb represents 5, and the other four fingers each represent 1. For example, 7+6, put your hand out as required, and you will know at a glance that 7+6= 13. After regular practice, students can see the formula, that is, present the shape of the hand in their minds, and with the help of the shape, students can calculate the numbers immediately. This can also be said to be a simple mathematical model. This is different from counting fingers, which is a low level, while the "two-handed memory method" belongs to group calculation and reaches a higher level. 4, abacus mental arithmetic. It is a new algorithm combining abacus calculation and mental calculation, which was founded by Mr. Chen, a math educator in his 80s. Its basic method is to calculate by hand code. The thumb of the right hand stands for 5, each finger of the rest stands for 1, and each finger of the left hand stands for 10. Use your mouth, heart and brain to calculate. 5, add up to ten methods. 8+5=8+2+3= 13, decompose the second addend and multiply the first addend by ten. 6. Formula method. On the basis of understanding the method of adding ten, memorize the formula of addition and directly report the result of 8+5= 13 (85+ 13). Several methods of abdication subtraction within 20: 1, counting method. Subtract one by one by counting your fingers or drawing ideas. For example, 12-4 is calculated with12-1-1. 2. Violation of the Ten Laws. Subtract the minuend from the minuend's ten digits and then add the minuend's one digit. For example, before 12-4 = 10-4+2 = 8, this method was used in primary school mathematics textbooks. 3. Inverse algorithm. According to the reciprocal relationship between addition and subtraction, subtraction is done by addition, and now some textbooks also use this method. The thinking process is simpler and faster than the "ten-break method". There is a premise to use the inverse algorithm: you must master the carry addition within 20. If it is a back addition formula, it is more advantageous to use the inverse algorithm, that is, to use the addition formula for subtraction. For example: 12—4=8 (8) 4 12 This method is also very convenient to use in pen subtraction, which can improve the calculation speed. 4, ten plus complement method. "Complement" refers to "complement" and refers to the number rounded to ten. For example, the complement of 9 is 1, and the complement of 8 is 2 ... The thinking process of this method is as follows:12-4 =12-10+6 = 8.