0.9+0.99+0.999+0.9999

= 1-0. 1+ 1-0.0 1+ 1-0.00 1+ 1-0.000 1

= 1+ 1+ 1+ 1-(0. 1+0.0 1+0.00 1+0.000 1)

=4-0. 1 1 1 1

=3.8889

Extended data

Simple operation of decimal point addition and subtraction

First, use legal methods.

Example1.3.82+2.79+6.18+7.21.

Analysis and solution: When calculating decimal addition, additive commutative law and associative law are often used for simple calculation.

Second, the method of removing brackets

Analysis and solution: the method of removing brackets often appears in the topic of the sum or difference of two numbers MINUS one number.

Third, the method of parenthesis.

Analysis and solution: The bracket method refers to appropriately adding brackets to the topic, changing the operation order of the original topic, so as to achieve the purpose of simple calculation.

Fourth, the displacement method

Analysis and solution: in the mixed operation of addition and subtraction, we can exchange the operation order of addition and subtraction (that is, 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.

V. Constant deformation method

Analysis and solution: When solving a problem by this method, the addend or subtrahend in the problem are usually close to integers such as 10 or 1. At this time, the constant deformation method can be used for deformation, so as to achieve the purpose of simple calculation.