Decimal times integer
1, decimal multiplication has the same meaning as integer multiplication: a simple operation to find the sum of several identical addends.
2. When a decimal is multiplied by an integer, the listed vertical forms should be aligned at the end first, and then the integer is multiplied by the integer. The decimal point of the product should be aligned with the decimal point of the factor, and the "0" after the decimal point should be crossed out.
3. Conversion related to time unit: 1 = 60 minutes, 1 minute = 60 seconds.
Decimal times decimal
1. When multiplying a decimal by a decimal, first expand the decimal into an integer, then multiply the integer by an integer to calculate the product, and then look at the factor-* * *. How many decimals are there? Just count a few decimal places from right to left at the end of the product and point the decimal point. Finally, the "0" after the decimal point should be crossed out.
Note: If the multiplied product has insufficient decimal places, it should be supplemented by 0 first, and then point to the decimal point.
2. Multiply several factors, and there are several decimals in the factor-* *, and the product has several decimals.
Relationship between products and factors
Multiply a number (except 0) by a number greater than 1, and the product is greater than the original number; -NUMBER (except 0) times a number less than 1, and the product is less than the original number; A number (except 0) is multiplied by a number equal to 1, and the product is equal to the original number.
Decimal multiplication estimation
When solving practical problems, we sometimes need to approximate the product according to the requirements of the topic. We use "rounding method" to approximate the decimal product.
1, rounding method
The last digit is less than 5: discard it and the digit on the right directly; The last number is greater than or equal to 5: first enter 1 in the previous number, and then discard it and the number on the right.
2. When solving practical problems with estimation, we should choose the appropriate estimation strategy according to the specific situation and data characteristics of the problem.
Judging "enough", all data should be estimated to be large or unchanged; Judging "insufficient", all data should be estimated to be small or unchanged; When estimating, we should also pay attention to moderation and effectively solve problems.
The changing law of products
1, the change of the factor will cause the change of the product.
If one factor is expanded to the original A times and the other factor is expanded to the original B times, the product will be expanded to the original a×b times. If one factor is reduced to 1/a and the other factor is reduced to 1/b, the product is reduced to 1/(a×b), which is equivalent to ÷(a×b). If one factor is expanded to the original a times and the other factor is reduced to the original 1/b, then the product should be × A first and then ÷b (in the above three cases, a and b are not 0).
2. The nature of product invariance
If one factor is enlarged to the original a times (a is not 0) and the other factor is reduced to the original 1/a, the product remains unchanged.
Decimal multiplication mixed operation
1, decimal elementary arithmetic is the same as integer elementary arithmetic.
2. The arithmetic of integers is also applicable to decimals, and some decimals can be easily calculated by using the arithmetic.
Multiplication reciprocity law: exchange the positions of two factors, and the product remains unchanged. Multiplication and association law: multiply the first two numbers, or multiply the last two numbers first, and the product is unchanged. Multiplication and distribution law: the sum of two numbers is multiplied by a number, which can be multiplied separately and then added.