First, use legal methods.
For example 1, 3.82+2.79+6.18+7.21.
Analysis: When calculating decimal addition, additive commutative law and associative law are often used for simple calculation. In this problem, 3.82, 6. 18, 2.79 and 7.2 1 can all add up to integer ten digits, so we can exchange the positions of 2.79 and 6. 18 and make a simple calculation by using the law of additive combination.
3.82+2.79+6. 18+7.2 1
=3.82+6. 18+2.79+7.2 1
=(3.82+6. 18)+(2.79+7.2 1)
= 10+ 10
=20
Second, the method of removing brackets
Example 2,
Example 3,
Analysis: the method of removing brackets often appears in the topic of the sum or difference of two numbers. Careful observation of Example 2 and Example 3 shows that? It can be rounded and simplified, so we can remove the brackets to simplify the calculation, but in the process of removing the brackets, we should pay attention to the change of symbols and turn the symbols in brackets into opposite symbols.
Third, the method of parenthesis.
Example 4,
Example 5,
Analysis: The parenthesis method refers to appropriately adding parentheses to the topic and changing the operation order of the original topic, so as to achieve the purpose of simple calculation. In example 4? Can be accumulated to the integer 1, as shown in Example 5. It can also add up to an integer of 1, so we can make a simple calculation with parentheses, and pay attention to the sign change caused by parentheses.
Fourth, the displacement method
Example 6,
Example 7,
Example 8,
Analysis: In the mixed operation of addition and subtraction, we can exchange the operation order of addition and subtraction (that is, the position) for simple calculation, which is the shift method. Because addition and subtraction are the same level operations, the exchange position does not affect the calculation results.
Careful observation shows that 8. 18+ 1.82 in Example 6 can add up to an integer 10. It can be accumulated to the integer 1, as shown in Example 8. An integer that can add up to 2. Therefore, the positions of addition and subtraction can be moved to simplify these three examples.
V. Constant deformation method
Example 9,
Example 10,?
Analysis: When solving a problem in this way, the addend or subtrahend in the problem are usually close to integers such as 10 or 1. At this time, the deformation can be carried out by the method of constant deformation, so as to achieve the purpose of simple calculation. In Example 9, 9.9 is close to 10, 9.9+0. 1= 10, and 9.9 is an addend. To ensure the same result, another addend will become? Convert the original formula into? , and then make a simple calculation; In the example of 10, 0.99 is close to 1, 0.99+0.0 1= 1, and 0.99 is a reduction. To ensure the same result, the minuend should become.