I. Significance
1, decimal times integer: a simple operation to find the sum of several identical addends.
For example, 3.2+3.2+3.2+3.2 is expressed as (3.2×5) by a multiplication formula, and the meaning of this multiplication formula is (how many are five 3.2).
2. Decimal times decimal: that is, how much is a fraction of this number.
For example, 1.5×0.8 is to find what is eight tenths of 1.5.
Second, arithmetic.
1, calculation method: calculate the product according to the law of integer multiplication, and then point the decimal point; When calculating the decimal point, the decimal point is calculated from the right side of the product according to the number of decimal places in the factor.
The calculation rules of decimal multiplication are simply: one number, two looks, three numbers, four points and five walks.
2. Note: In the calculation result, the 0 at the end of the decimal part should be removed to simplify the decimal; When the number of decimal places is not enough, use 0 to occupy the place.
3. There are many ways to check multiplication: you can exchange the positions of two factors and recalculate; Estimation method can be used; You can also use a calculator to check.
4, the relationship between products and elements:
A number (except 0) is multiplied by a number greater than 1, and the product is greater than the original number.
A number (except 0) is multiplied by a number less than 1, and the product is less than the original number.
Expressed in letters: a×b=c(a is not equal to 0).
B> 1, a> C.
b= 1，a=c .
b & lt 1，a .
Third, the approximate quantity of products.
1, there are three methods to find the divisor: rounding method, forward method and rounding method, and rounding method is mainly used in this unit.
The steps are as follows: first, calculate the product by decimal multiplication, and then take the approximate value according to the requirements of the topic and the "rounding" method.
Note: When representing approximate values, the 0 after the decimal point cannot be removed casually.
For example, 0.599 with two decimal places is ().
2. In general, the smallest unit of RMB is minutes, and the decimal in yuan represents percentile.
Fourth, mixed operation.
The order of decimal four operations is the same as that of integers.
The commutative law, associative law and distributive law of integer multiplication are also applicable to fractional multiplication.
Simplification of multiplication and division is the key and difficult point in this part.
Case: 0.25×4.78×4.
0.65×202。
2.4× 1.5-2.4。
2.4×0.6+2.6×0.6。
12.5×32×0.25。
Verbs (short for verb) solve problems.
1. The estimation application in real life can be large or small, and the appropriate estimation strategy should be selected according to the actual situation.
2. The problem of subsection charging, such as taxi and water and electricity charges, belongs to subsection charging. There are two solutions: the first is to merge after charging in sections; The second method is to calculate the whole unit price, and then add the undercalculated amount.