1. Combined with the specific situation, we can correctly estimate the addition and subtraction of three digits. (Teaching emphasis)
2. In the process of solving practical problems, we will choose appropriate methods to estimate addition and subtraction. (difficulty)
3. Feel the application of estimation in daily life, and develop estimation ability and sense of number.
Knowledge map;
Solving problems through estimation can be divided into two parts: estimation method and selection of estimation strategy according to actual situation.
1. Estimation method:
A treat each three-digit number as an integer estimate close to it.
B. treat each three-digit number as an estimate of hundreds of digits nearby.
2. Choose the estimation strategy according to the actual situation: estimate big or small; Pay attention to reason.
Instructional design:
First, create a situation
In the Expo Park, there are not only various exhibition halls, commemorative shops, but also giant screen cinemas! Let's have a look!
1. What mathematical information did you get? Answer by name
Grade one to grade three: 223.
Grade 4-6: 234.
Maximum image: 44 1 person (briefly introduce the maximum image)
There are quite a few students who come to the movies! Can so many students sit and watch the biggest image at the same time? how do you know
Second, explore new knowledge.
1. Thinking analysis
To know whether students of six grades can sit at image maximum at the same time, we should compare the total number of students of six grades with the number of seats in the giant screen cinema:
The total number of students in six grades is 223+234.
If 223+234 is greater than 44 1, you can't sit down;
If 223+234 is less than or equal to 44 1, then you can sit down.
Because the question is whether you can sit down, you don't need to get the exact value. You can estimate the result of 223+234 first, and then compare the estimated result with 44 1.
Solve problems with estimates
The addend is estimated to be a few hundred dozen close to it, that is, 223 is 220 and 234 is 230.
223 + 234 =
220+230 = 450 is greater than 44 1.
Because 223 is greater than 220, 234 is greater than 230, and 220+230 = 450, 223+234 must be greater than 450, that is, you can't sit in the right place.
Review and reflection
Small estimation method: it is a small estimation method that estimates all known data to the minimum, the calculated result is still greater than the value to be judged, and the explanation is not enough or it cannot be loaded.
Answer correctly
Because 223 "220, 234" 230, 220+230 = 450 (person),
So 223+234 must be greater than 450, so you can't sit.
A: Students in Grade Six can't sit down while watching image maximum.
Third, autonomous migration and practical application.
Show me the question:
The giant screen cinema has 44 seats/kloc-0 seats. If two tour groups have 196 and 226 members respectively, can both tour groups see image maximum at the same time?
Students have just learned similar content and have a certain foundation. At this time, they can let the students migrate and use their autonomous learning-first let the students learn independently (autonomous migration), then discuss and share their own methods in groups, and finally the group representatives will come to the stage to explain and share.
Estimation method: the estimation method is to estimate all the known data, and the calculated result is still less than the judgment value, indicating that it is enough or can be loaded.
Correct answer:
196 《 200 226 《 230
200+230 =430 (person) 430 "44 1 "
So 196+226 must be less than 44 1, so you can sit down.
A: These two tour groups can see the maximum image at the same time.
Fourth, summary.
What do we mainly study in this course? Please tell me what you have learned-solving problems with estimation (blackboard project)
1. estimation is not the rounding of exact numbers, but an important strategy to solve problems.
2. When solving problems with estimation, we should first judge whether the estimation is large or small, that is, we should pay attention to its rationality.