Properties of Multiplication

Properties of Operations are those three pesky words that reoccur throughout the common core standards.  Some of you may be wondering exactly what they mean.  Properties of Operations are the foundation of arithmetic; we use them when performing computations and recalling basic facts. 

In this post, I will focus on the following 3 properties that are used with addition and multiplication:

  • Commutative Property
  • Associative Property
  • Distributive Property
Let’s take a closer look at how the properties relate to multiplication. 
properties of multiplication
The commutative property states that changing the order of the factors does not change the product.  The root word of commutative is commute or interchange (refer to the chart below). I know that maybe too much information…… but I thought it was interesting.
photos of commutative property, associative property, distributive property, mr elementary math
The commutative property can be very confusing to students, such as in the example 8 x 2 = 2 x 8.  Although both equations represent the same amount or product, representing them with a visual model looks different (eight groups of two versus two groups of eight).  See below.
 
photos of commutative property, mr elementary math

An idea to help with the commutative property may be:

  • Providing students with tiles or counters and asking them to model 8 x 2 & 2 x 8;  3 x 4 & 4 x 3,  5 x 3 & 3 x5.  Then ask the students to compare the product of each factor pair (ie. 8 x 2 and 2 x 8).  Students should be able to explain that the products are identical.  To sum it up, students should model, model, model multiplication equations in any order to see if the products are always identical.
Screen-Shot-2013-08-04-at-8.23.21-PM1

Next, the associative property states that changing the grouping of the factors does not change the product. This property works closely with the commutative property because we often change the order of groupings of factors when multiplying numbers to make it easier to solve problems.

Using 3 x 2 x 2 as the basis of our groupings, below you will see a visual model of how the associative property works.

An idea to help with the associative property may be:
 
  • Providing the students with counters or tiles and asking them to model (3 x 2) x 2 and then 3 x (2 x 2); like the example above.  From this point allow the students to determine patterns they notice.  Consider asking the students: What is the product of both expressions?  Why do they think the product is the same for both? Try this investigation with different expressions including:  (5 x 2) x 2 and 5 x (2 x 2), (3 x 4) x 2 and 3 x (4 x 2).  Students should begin to generalize that changing the groupings does not change the product. 
Properties of multiplication
Last but certainly not least is the distributive property. The distributive property basically lets us spread out the factors so that the numbers are easier to work with.  We use this a lot when multiplying mentally.  For example, if asked to mentally find the product of 54 x 3, many of us would decompose 54 into 50 and 4.  We could then say that (50 x 3) + (4 x 3) = 150 + 12 = 162.  
 
Check out the visual model of distributive property below:
photos of distributive property, mr elementary math
An idea to help with the distributive property may be:
 
  • Asking students to model 2 groups of 6 (2 x 6) using connecting cubes and find two hidden facts inside of 2 x 6 like 2 x 3 and 2 x 3. Allow the students time to discover as many hidden facts inside of the 2 x 6 that they can.  Try this out with different multiplication facts through 10 x 10. Do not forget to ask students what they notice as they are doing this investigation.

 

 

photos of distributive property, mr elementary math

Worked examples

  • Simplify 3a – 5b + 7a. Justify your steps.

I’m going to do the exact same algebra I’ve always done, but now I have to give the name of the property that says its okay for me to take each step. The answer looks like this:

3a – 5b + 7a original (given) statement
3a + 7a – 5b Commutative Property
(3a + 7a) – 5b Associative Property
a(3 + 7) – 5b Distributive Property
a(10) – 5b simplification (3 + 7 = 10)
10a – 5b Commutative Property

 

The only fiddly part was moving the “– 5b” from the middle of the expression (in the first line of the table above) to the end of the expression (in the second line). If you need help keeping your negatives straight, convert the “– 5b” to “+ (–5b)“. Just don’t lose that minus sign!

 

 

 

    • Simplify 23 + 5x + 7y – x – y – 27.   Justify your steps.
23 + 5x + 7y – x – y – 27 original (given) statement
23 – 27 + 5x – x + 7y – y Commutative Property
(23 – 27) + (5x – x) + (7y – y) Associative Property
(–4) + (5x – x) + (7y – y) simplification (23 – 27 = –4)
(–4) + x(5 – 1) + y(7 – 1) Distributive Property
–4 + x(4) + y(6) simplification
–4 + 4x + 6y Commutative Property
    • Simplify 3(x + 2) – 4x.   Justify your steps.
3(x + 2) – 4x original (given) statement
3x + 3×2 – 4x Distributive Property
3x + 6 – 4x simplification (3×2 = 6)
3x – 4x + 6 Commutative Property
(3x – 4x) + 6 Associative Property
x(3 – 4) + 6 Distributive Property
x(–1) + 6 simplification (3 – 4 = –1)
x + 6 Commutative Property

Practice Questions

Why is it true that 3(4 + x) = 3(x + 4)?

 

 

Why is 3(4x) = (3×4)x?

 

 

 

Why is 12 – 3x = 3(4 – x)?

 

 

 

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