Use the "ten-complement method" to calculate orally.

According to the characteristics of the formula, apply laws and properties to "round" the operation data:

1, and the addend "round off".

For example, 14+5+6 =? Enlighten students: Add up several figures. If several numbers add up to an integer 10, you can change the position of the addend and add up several numbers.

2. Use the nature of subtraction to "round off".

For example, 50- 13-7, to inspire students to tell their thinking process, tell several oral calculation methods and compare them, so that students can draw a conclusion: if you subtract several numbers from a number continuously, if the sum of subtraction can add up to an integer, you can add the subtraction first and then subtract. This kind of oral calculation is relatively simple.

3. Multiplication factor "rounding".

Such as 25× 1? The product of 4× 4, 25 and 4 is 100, and the result can be calculated directly by mouth.

two

Use "decomposition method" for oral calculation

It is to "disassemble" a number in the topic and calculate it with another number, such as 2? 5×32, the original formula becomes 2? 5×4×8= 10×8=80。

three

Do oral calculations with some quick calculation skills.

1. Two digits are quickly multiplied by the first and last digits 10.

That is, multiply the number on one of the ten digits by 1, and the product is one hundredth and one thousandth of the product of two numbers, and then the product of the numbers on two numbers is one and one tenth of the product of two numbers. For example, 14× 16 = 224 (4× 6 = 24 is a unit, 10 digits, (1+ 1) × 1 = 2 is a hundred digits).

2. Fast multiplication of two digits of head difference 1 and tail difference 10. That is to say, the square of the ten digits of the larger factor MINUS the square of its single digits. Such as: 48× 52 = 2500-4 = 2496.

3. Use the "benchmark number" for quick calculation.

For example, 623+595+602+600+588 can choose 600 as the base number, first accumulate the difference between each number and the reference number, and then add the product of the base number and the number of items.

4. Master some operating rules.

For example, if both denominators are prime numbers and the numerator is 1, the product of denominator multiplication can be taken as the denominator and the difference between denominators as the numerator; Both denominators are prime numbers with the same numerator. You can take the product of denominator multiplication as denominator, then multiply the difference of denominator subtraction by numerator as numerator, and so on.