Уроки математики и физики (RU + UA)

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понедельник, 13 июля 2015 г.

Lesson 3. Substracting whole numbers

You subtract to tell how many are left. This is called the difference.

EXAMPLE:
You add to tell how many it all. You subtract to tell how many are left. When you know an addition fact, you know a subtraction fast too.  

 EXAMPLE:
Sometimes you can solve a word problem by subtracting.

EXAMPLE:
Some problems take two steps to solve. You may have to subtract first. Then you use your answer to add. The sum is the answer to the problem.  

EXAMPLE:
The subtraction of two numbers  a  and  b  is denoted by

a – b = c,

where  c  is the result of the subtraction of  b  from  a. The result  c  is also called the difference between  a  and  b, and  a  is known as the subtrahend.
One can easily check whether the result of the subtraction is true. Indeed, by the definition, the difference  a – b  is the number  c  that by which the number  b  is added to produce  a:

c = a – b              
a = b + c

When you add or subtract whole numbers, line up the numbers by place value. Then treat each place value as its own addition or subtraction problem.
EXAMPLE:

To subtract 2564 from 2723, line up the numbers by place value. Subtract each place value, starting in the ones place. Since you cannot subtract 4 ones from 3 ones, you have to regroup. Barrow one ten from the tens place to make the 3 ones into 13 ones, in this example, you also have to borrow one hundred from the hundreds place.
Problem solving
Fist, make sure to read the entire problem before you begin to work.
Second, identify the problem and the skill required to solve it. (Is it addition or subtraction, or both?)
Third, decide what information is needed to solve the problem. Identify any information that is not needed and ignore it.
Fourth, begin to work the problem. Fifth, check your answer.

EXAMPLE:

Jason worked 156 hours this month. Marc worked 171. How many more hours did Marc work that Jason?
Step 1:
What skill is needed? Subtraction
Step 2:
Subtract Jason’s hours from Marc’s hours.
Step 3:
171 – 156 = 15
Step 4:
Marc worked 15 more hours than Jason. 

Tasks to the lesson 3
Other lessons:
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