8+2×7 9×3+2×3 18÷3-4

8 1÷9×2 16+3×4 56÷8-2

7×6- 10 38-5×5 3×9÷3

24÷4×3 100÷4-20 20-20÷5

Please explain the operation order of the last oral test "100÷5×3". (First calculate 100÷5 equals 20, and then multiply it by 3)

Learn a new course

Example 1: Calculate 74+ 100 ÷ 5× 3.

Example 1: Calculate 74+ 100 ÷ 5× 3, and think as follows: (Projection)

This problem involves several levels of operation.

(2) What kind of operation sequence should be adopted in calculation?

(3) What is the first? What is it? What is it?

On the basis of personal independent thinking, discuss with classmates at the same table, and then try on your own exercise book. (Some students wrote it on the glass)

Please explain the calculation process when modifying.

74+ 100÷5×3

=74+20×3

=74+60

= 134

On the basis of the students' answers, the teacher gives specific guidance.

This is an elementary arithmetic problem without brackets. You should calculate multiplication and division first, and then add and subtract. In the continuous calculation of multiplication and division, you should calculate from left to right. In the process of separation, you should draw the operation sequence line according to the operation sequence, and you should also do the "three checks". First, you should check whether the numbers and symbols copied from the book to the exercise book are correct. Second, you should check the numbers in the draft from horizontal to vertical.

Oral calculation: 500-400 ÷ 4

(500-400)÷4

What's the difference between these two questions? (The operation sequence is different) Why? (The second question is bracketed, which changes the operation order. )

Example 2:

Calculate (440-280) × (300-260)

Teacher: There are two brackets around this question. How should I calculate it? (Someone wrote it on the glass)

Let the students try it themselves, and the following two situations may occur.

( 1)(440-280)×(300-260) (2)(440-280)×(300-260)

= 160×(300-260) = 160×40

= 160×40 =6400

=6400

When reviewing, guide the students to discuss.

Teacher: Students have the above two forms of off-balance-sheet calculation. (1) The problem is to carry out unbalanced calculation step by step.

(2) The problem is the simultaneous calculation of subtraction in two brackets. Both disjunctive forms are correct, so we can compare them. Which method of disjunctive calculation is simple? Why?

Through the discussion between teachers and students, it is concluded that it is easier to calculate with two brackets at the same time.

The teacher is writing on the blackboard:

(440-280)×(300-260)

= 160×40

=6400

Do it:

( 1)65-6×4÷2

(2)38+56÷7×3

(3)(59+2 1)×(96÷8)

(4)(220- 100)÷( 15×2)

When revising, please state the operation order of each question.

(3) Integrated feedback

1. Tell the sequence of the following questions. (projection)

( 1)700-8×5×4

(2)840÷6÷7+630

(3)( 15×40-360)÷6

(4)(26+ 19)×(49÷7)

2. judge. (Prepare "√" and "×" feedback cards) (Forecast)

( 1) 45+55÷5-20 (2) 130+60-90×2

= 100÷5-20 = 190-90×2

=20-20 = 100×2

=0 =200

( ) ( )

(3)48+20÷4×5 (4)320- 15×4+40

=48+20÷20 =320-60+40

=48+ 1 =200- 100

=49 =200

( ) ( )

3. Fill in the number in □, and then list the comprehensive formula. (projection)

4. List the comprehensive formula according to the following two groups of topics. (projection)

( 1)96÷8= 12 (2) 12+24=36

12+ 18=30 36÷9=4

84-30=54 4×5=20

Line: _ _ _ _ _ _ Line: _ _ _ _ _ _

5. In the following formula, add brackets appropriately to make the equation hold.

( 1) 12×6+8÷4=20

(2) 12×6+8÷4=42

(3) 12×6+8÷4=96

Teachers and students sum up together.

The mixed operation we learned today is three steps, and the three-step mixed operation has two brackets. Therefore, when calculating the mixed operation problem, just like solving an application problem, we should first examine the problem, see which operations it contains, whether there are parentheses, and decide what to calculate first and then what to calculate. After calculation, we should check.

Homework: Page 921_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _.

2.________ ________ ________ ________

Small data [four operation sequences]

Among the four operations, addition and subtraction are called primary operations, and multiplication and division are called secondary operations. Expressions that contain two or more operations are usually called mixed operations. The mixed operation of addition, subtraction, multiplication and division is also called elementary arithmetic. In elementary arithmetic, the prescribed calculation order is called operation order. Mathematically, the four operation sequences are as follows:

(1) In an expression, if only operations of the same level are included, operations should be performed from left to right. In other words, it only contains addition and subtraction, or only a mixed operation of multiplication and division, and their operation order is calculated from left to right.

(2) If an expression contains both first-level operation and second-level operation, then the second-level operation should be calculated first, that is, "multiply and divide first, then add and subtract", or "multiply and divide first, then add and subtract" for short.

(3) If you want to change the operation order mentioned above, you need to use parentheses. There are three kinds of brackets commonly used: parentheses, marked as (); Parentheses, marked []; Braces, marked {}. When using parentheses, use parentheses first, then brackets, and finally braces.

If a formula contains several brackets, it should be counted in brackets first, then in brackets, and finally in braces. When calculating, the formulas in brackets should be calculated in the order mentioned above, and then the results and the numbers outside brackets should be calculated in the same order.

Description of classroom teaching design