1.？ Be able to use what you have learned to solve practical problems and feel the close connection between digital operation and life.

2.？ Cultivate the initial awareness of estimation and the ability to choose appropriate methods to solve simple practical problems.

3.？ In the process of solving problems, experience the fun of learning and stimulate the interest in learning mathematics.

Teaching content:

First of all, go straight to the topic and make a preliminary analysis and estimation.

Teacher: "Students, do you remember what an estimate is? What is the estimate? "

Health 1: "Estimation is based on one thing to guess another. Just like last time, according to the first layer of a box of chess pieces on the left is about 10, we can guess that there are five layers on the right, which is about 50 pieces. "

Health 2: "I guess I just think about it."

Health 3: "I guess it's not a random guess, it must be an informed guess."

Teacher: "That's good. Now, please look at this picture and tell me what the relationship between these two baskets of apples is. "

Health 1: "The apple on the right is half on the left."

Health 2: "There are about two apples on the left."

Teacher: "If there are about 15 apples on the left, guess how many apples are on the right?"

Health: "30", "3 1" and "28"

Teacher: "Do you agree with them?"

Health: "agree"

Teacher: "Why?"

Health: "Because it's probably more than half."

Second, create scenarios and explore the significance of estimation.

Teacher: "Today, the teacher told you a story about estimation, which is a true thing. I have a brilliant grandfather. Although he has never been to school, his mathematical calculation ability is really first-class. Do you know why? "

I watched and listened quietly.

I continued to preach, "Grandpa went to the supermarket to buy fruit. He bought a big bag of apples, 8 yuan, a big bag of pears and 9 yuan. But grandpa only brought 20 yuan. Is this enough? "

Health: "That's enough"

Teacher: "Do you know what Grandpa thinks?"

Health: "8+9 = 17 17 < 20"

The teacher shook his head and said, "Please listen to grandpa. It must be enough. If 20 yuan is divided into 10 yuan and 10 yuan, there are not enough apples in 8 yuan and not enough pears in 9 yuan. Neither is more than 20 yuan. It must be enough. Which one of you can understand grandpa's algorithm? "

Health: "Grandpa compared 10 yuan with 8-yuan apples and 9-yuan pears, and none of them exceeded 10 yuan, so together they definitely did not exceed 20 yuan."

Teacher: "In fact, Grandpa just estimated both 8 and 9 as an integer decimal system like 10, which makes the calculation much more convenient."

Teacher: "What else surprises me? Aunt clerk especially likes to ask our children when checking out. How much should aunt give you? This question stumped me, when I was still struggling to calculate 8+9=? Then use 20- 17=? At this time, grandpa smiled and said, I'll teach you. Guess what grandpa would do, so I admire him so much? "

Sheng shook his head, and I went on to say, "Just now, 10 yuan paid for apples, 10 yuan paid more for pears than 2 yuan, and 1 yuan has to be recovered, so 3 yuan has to be recovered."

Health: "this is too fast!" " "

Teacher: "We just need to estimate 8 and 9 as 10 and analyze their extra parts."

Teacher: "have you learned grandpa's method?" Then let's challenge this formula 17+ 15=? For this formula, it is more or less difficult or error-prone to use the methods we have learned before. Today we will try grandpa's method. "

Teacher: "Can we take 17 as?"

Health: "20 whole tens"

20+ 15 is much easier to calculate, 20+ 15=35, but it was originally 17, which was estimated to be 20 and overestimated by 3, so the actual result should be 3 less, so exit 17+ 15=32.

Teacher: "It seems much more convenient to estimate a complex number as an integer close to it."

Third, explore independently and deepen understanding.

Teacher: "Naughty and Xiaoxiao are picking apples in the orchard. Let's see how much they picked? "

Teacher: "What mathematical information did you extract from the picture?"

Health: "Naughty picked 17 apples, and Xiaoxiao picked 20 apples. A box can hold 60 apples. "

Teacher: "What does 60 mean on this box?"

Health: "That is, there can be 60 apples in the box."

Teacher: "Then I have 6 1 apple. Can I hug them? "

Health: "No"

Teacher: "Then I have 59 apples. Can I hug them? "

Health: "Yes"

Teacher: "Be precise. What does it mean to mark this box with 60 pieces? "

Student: "You can only hold 60 at most."

Teacher: "Then according to these mathematical information, what do you want to know most now?" What questions do you want to ask? "

Student: "Can this box hold?"

Teacher: "That's good. I also want to know. What do we need to know first to know if these apples can be installed? "

Health: "How many apples are there?"

Teacher: "How to arrange it?"

Health: "38+ 17="

Teacher: "This formula is not easy to calculate at all. Let's try it by estimation. Can we adapt? "

Student report and teacher summary:

(1) estimated that 38 is 40, 17+40=57, 57 < 60, and the result is less than 60, so it can be loaded. We can put 38 into 40, but in fact we can put it into it.

(2) It is estimated that 17 is 20, 20+38 = 58, 58 < 60, and the result is less than 60, so it can be loaded. We estimate that 17 can hold 20, but it can actually hold more.

(3) Think of 38 as 40, 17 as 20, which is too much, and 40+20=60 can be fitted. We estimate that 17 and 38 can be installed, but in fact they can be installed.

Teacher: It seems that it is convenient to calculate with integer ten digits. Calculating 17 and 38 into close integer tens like this is called estimation.

Fourth, use estimates to calculate the results.

Teacher: "Can we calculate the actual result by estimation?" Let's try it together! "

Guide students to discuss:

(1) If 38 is estimated to be 40 and two more are estimated, then the estimated result 57 is also estimated by two more, and the actual result is missing by two, that is, 55.

(2) It is estimated that 17 is 20. If you estimate more than 3, the estimated result will be more than 3, and the actual answer will be less than 3, that is, 58 and less than 3 are both 55.

(3) estimate 17 into more than 20, estimate 3; If you estimate 38 as more than 40 and 2, then you estimate 5 more, and the actual answer will be 5 less, that is, 60 and 5 less are 55.

Summary: We found that in the process of calculating "17+38=", whether we estimate only one number or all numbers, the final calculated answer is the same. It seems that estimation can not only help us solve simple problems, but also simplify calculations and make it easier to add and subtract complex two-digit numbers.

Five, apply what you have learned and consolidate new knowledge

1、 ? Class 1 (1) has 48 students and Class 1 (2) has 47 students. The school organized two classes to visit the museum and rented a bus for 90 people. Is it okay?

2. Calculate the following formula by estimation method.

19+23=32+46= 85+9= 52—8= 100-43=? 48—9=