Hugging: In simple addition and subtraction, the basic idea is "rounding". According to the properties of commutative law, associative law and subtraction of addition (multiplication), if any of them can be rounded, the formula can be changed, so that the rounded numbers can form a pair of good friends and the rounding calculation can be simplified.

For example: 298+304+ 196+502

Analysis: In this question, additive commutative law and the law of association can be used to add the numbers that can be made into whole ten, whole hundred and whole thousand, which can make the calculation simple.

Answer: The original formula = (298+502)+(304+196) = 800+500 =1300.

2. Quick calculation and clever calculation of chess signing.

Instruction: In addition to addition, subtraction, multiplication and division, the positions of numbers can be exchanged according to the needs of operation and the characteristics of the topic, which can make the calculation simple. Special reminder: exchange the positions of numbers, and note that the operation symbols also change positions.

For example: 464-545+836-455

Analysis: Observing the example, you will find that if you want to calculate from left to right according to the convention, 464 MINUS 545 is not enough at all. In primary school, students can't do it, so to do this problem, students should first observe the characteristics of numbers and make simple calculations.

Thinking: Can 4.75÷0.25-4.75 move with the symbol? When can I move with a sign? sign

3. Quickly calculate and skillfully calculate the rounded split score.

Pointing: According to the law of operation and the characteristics of numbers, the numbers in formulas are often divided and recombined flexibly to make up whole ten, whole hundred and whole thousand respectively.

For example: 998+ 14 13+9989.

Analysis: 998 plus 2 can be added to 1000, 9989 plus 1 1 can be added to 10000, so we divide 14 13 into 1400.

The solution formula is = (998+2)+1400+(11+9989) =1000+1400+/0000 =/kloc.

For example: 73. 15×9.9

The analysis regards 9.9 as the difference of 10 minus 0. 1, and then multiplies the distribution rate to simplify the operation.

The formula = 73.15× (10-0.1) = 73.15×10-73.15× 0./kloc-.