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1.9: Properties of Real Number Addition

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On the first day of school, you are all dressed in your new clothes. When you got dressed, you put one sock on your left foot and one sock on your right foot. Would it have made a difference if you had put one sock on your right foot first and then one sock on your left foot?

Guidance

There are five properties of addition that are important for you to know.

Commutative Property

In algebra, the operation of addition is commutative. The order in which you add two real numbers does not change the result, as shown below:

(+7)+(+20)&= ? && (+20)+(+7)=?\\(+7)+(+20)&={\color{blue}+27} && (+20)+(+7)={\color{blue}+27}

The order in which you added the numbers did not affect the answer. This is called the commutative property of addition. In general, the commutative property of addition states that the order in which two numbers are added does not affect the sum. If a and b are real numbers, then a+b=b+a .

Closure Property

The sum of any two real numbers will result in a real number. This is known as the closure property of addition. The result will always be a real number. In general, the closure property states that the sum of any two real numbers is a unique real number. If a, b and c are real numbers, then a+b=c .

Associative Property

The order in which three or more real numbers are grouped for addition will not affect the sum. This is known as the associative property of addition. The result will always be the same real number. In general, the associative property states that order in which the numbers are grouped for addition does not change the sum. If a, b and c are real numbers, then (a+b)+c=a+(b+c) .

Additive Identity

If zero is added to any real number the answer is always the real number. Zero is known as the additive identity or the identity element of addition. The sum of a number and zero is the number. This is called the identity property of addition. If a is a real number, then a+0=a .

Additive Inverse

The sum of any real number and its additive inverse is zero. You learned in example D that zero is also called the identity element of addition. Therefore, it can be said that the sum of any real number and its additive inverse is the identity element. This is called the inverse property of addition. If a is a real number, then a+(-a)=0 .

Example A

Use a number line to show that (5)+(-3)=(-3)+(5) .

On a number line, you add a positive number by moving to the right on the number line and you add a negative number by moving to the left on the number line.

(5)+(-3)=+2 The red dot is placed at +5. Then the (-3) is added by moving three places to the left. The result is +2.

(-3)+(5)=+2 The blue dot is placed at -3. Then the (+5) is added by moving five places to the right. The result is +2.

Example B

Does (-6)+(-2)= a real number?

(-6)+(-2)=-8 The red dot is placed at -6. Then the (-2) is added by moving two places to the left. The result is -8.

The result is -8. This is an integer. An integer is a real number. This is an example of the closure property.

Example C

Does (-4+7)+5=-4+(7+5) ?

(-4+7)+5= The red dot is placed at -4. Then the (+7) is added by moving seven places to the left. Then (+5) is added by moving five places to the right. The result is +8.

-4+(7+5)= The blue dot is placed at +7. Then the (+5) is added by moving five places to the left. Then (-4) is added by moving four places to the left. The result is +8.

(-4+7)+5=-4+(7+5)

The numbers in the problem were the same but on the left side of the equal sign, the numbers -4 and +7 were grouped in parenthesis. The addition in the parenthesis was completed first and then +5 was added to that sum. The final result was +8.

On the right side of the equal sign, the numbers +7 and +5 were grouped in parenthesis. The addition in the parenthesis was completed first and then (-4) was added to the sum. The final result was +8. This is an example of the associative property of addition.

Example D

Does (-5)+0=-5 ?

(-5)+0=-5 The red dot is placed at -5. If zero is being added to the number, there is no movement to the right and no movement to the left. Therefore the result is -5. This is an example of using the additive identity.

Example E

Does (+6)+(-6)=0 ?

(+6)+(-6)=0 The red dot is placed at +6. Then the (-6) is added by moving six places to the left. The result is 0.

When any real number is added to its opposite, the result is always zero. If a is any real number, its opposite is -a . The opposite, -a , is also known as the additive inverse of a .

Concept Problem Revisited

Think back to the question about putting on socks. The order in which you put on the socks does not affect the outcome – you have one sock on each foot.

This is like the commutative property in algebra. The order in which you add two real numbers does not change the result.

Vocabulary

Additive Identity
The additive identity for addition of real numbers is zero.
Additive Inverse
The additive inverse of addition is the opposite of the real number and the sum of the real number and its additive inverse is zero. If a is any real number, its additive inverse is -a .
Associative Property
The associative property of addition states the order in which three or more real numbers are grouped for addition, will not affect the sum. If a, b and c are real numbers, then (a+b)+c=a+(b+c) .
Closure Property
The closure property of addition states that the sum of any two real numbers is a unique real number. If a, b and c are real numbers, then a+b=c .
Commutative Property
The commutative property of addition states that the order in which two numbers are added, does not affect the sum. If a and b are real numbers, then a+b=b+a .
Identity Element of Addition
The identity element of addition is another term for the additive identity of addition. Therefore, the identity element of addition is zero.
Identity Property
The identity property of addition states that the sum of a number and zero is the number. If a is a real number, then a+0=a .
Inverse Property
The inverse property of addition states that the sum of any real number and its additive inverse is zero. If a is a real number, then a+(-a)=0 .

Guided Practice

1. Add using the properties of addition: -1.6+4.2+1.6

2. What property justifies the statement? (-21+6)+8=-21+(6+8)

3. Apply the commutative property of addition to the following problem. 17x-15y

Answers:

1.

& -1.6+4.2+1.6\\& -1.6+1.6+4.2 \rightarrow {\color{blue}\text{Communicative Property}}\\& (-1.6+1.6)+4.2 \rightarrow {\color{blue}\text{Associative Property}}\\& 0+4.2 \rightarrow {\color{blue}\text{Inverse Property}}\\& =4.2 \rightarrow {\color{blue}\text{Identity Property}}

2. (-21+6)+8=-21+(6+8)

The numbers on each side of the equal sign are the same but they are not grouped the same.

&(-21+6)+8 && -21+(6+8)\\&=(-15)+8 && =-21+(14)\\&=-7 && =-7

The order in which the numbers were grouped did not affect the answer. The property that is being used is the associative property of addition.

3.

& 17x-15y\\& 17x+(-15y) \ \mathbf{Rewrite \ the \ statement \ as \ an \ addition \ statement.}\\& 17x+(-15y)=-15y+17x

The commutative property of addition states that the order in which two numbers are added, does not affect the sum.

Practice

Match the following addition statements with the correct property of addition.

1. (-5)+5=0

2. (-16+4)+5=-16+(4+5)

3. -9+(-7)=-16

4. 45+0=45

5. 9+(-6)=(-6)+9

a) Commutative Property

b) Closure Property

c) Inverse Property

d) Identity Property

e) Associative Property

Add the following using the properties of addition:

  1. 24+(-18)+12
  2. -21+34+21
  3. 5+\left(-\frac{2}{5}\right)+\left(-\frac{3}{5}\right)
  4. 19+(-7)+9
  5. 8+\frac{3}{7}+\left(-\frac{3}{7}\right)

Name the property of addition that is being shown in each of the following addition statements:

  1. (-12+7)+10=-12+(7+10)
  2. -18+0=-18
  3. 16.5+18.4=18.4+16.5
  4. 52+(-75)=-23
  5. (-26)+(26)=0

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Date Created:

Dec 19, 2012

Last Modified:

Apr 29, 2014
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