Have you ever been skiing?

In Alaska, Molly and her family went on a skiing trip. There are four people in Molly's family. When they got there, they saw that four other families of four were also there. Molly isn't sure how much a lift ticket costs. This is an unknown variable in this situation.

Can you write a variable expression involving integer multiplication to figure out how many people will need lift tickets?

**Well, if you aren't sure how to do this, don't worry, you will learn all about it in this Concept.**

### Guidance

**Do you know what a variable expression is?**

**A** *variable expression***is a math phrase using numbers, operations and variables.** A variable expression can also contain like terms. A like term is a term that is common between one or more terms in the equation. When you have like terms, we can combine them using addition and subtraction.

**Pay attention! Here is a change!**

**How does this happen?**

Find the value of this expression \begin{align*}3z \cdot (-2)\end{align*}

**The** *Commutative Property of Multiplication***states that the order in which factors are multiplied does not matter.**

\begin{align*}3z \cdot (-2) = -2 \cdot 3z\end{align*}

**The** *Associative Property of Multiplication***states that you can group the factors being multiplied in any order.**

\begin{align*}-2 \cdot 3z = -2 \cdot 3 \cdot z = (-2 \cdot 3) \cdot z\end{align*}

**Now we can multiply the integers. Since -2 and \begin{align*}3z\end{align*} have different signs, the product will be negative.**

\begin{align*}2 \cdot 3=6\end{align*}, so \begin{align*}(-2)\cdot 3=-6\end{align*} and \begin{align*}(-2 \cdot 3)\cdot z = -6z\end{align*}

**The value of the expression is \begin{align*}-6z\end{align*}.**

Find the value of this expression \begin{align*}(-5)(-2m)(n)\end{align*}

**The Associative Property of Multiplication states that you can group the factors being multiplied in any order.**

\begin{align*}(-5)(-2m)(n) = (-5)\cdot (-2) \cdot m \cdot n\end{align*}

**Now we can multiply the integers. Since -5 and -2 are both negative, the product will be positive.**

\begin{align*}-5 \cdot -2=5 \cdot 2 = 10\end{align*}, so \begin{align*}(-5) \cdot (-2) \cdot m \cdot n = 10 \cdot mn=10mn\end{align*}

**The value of the expression is \begin{align*}10mn\end{align*}.**

Multiply the following variable expressions.

#### Example A

\begin{align*}3x(4y)\end{align*}

**Solution:\begin{align*}12xy\end{align*}**

#### Example B

\begin{align*}-6a(-4b)\end{align*}

**Solution:\begin{align*}24ab\end{align*}**

#### Example C

\begin{align*}-4z(10)\end{align*}

**Solution:\begin{align*}-40z\end{align*}**

Here is the original problem once again.

In Alaska, Molly and her family went on a skiing trip. There are four people in Molly's family. When they got there, they saw that four other families of four were also there. Molly isn't sure how much a lift ticket costs. This is an unknown variable in this situation.

Can you write a variable expression involving integer multiplication to show how many people will need lift tickets?

To write this variable expression, let's write down what we know first.

We know that there are four people in Molly's family who will all need lift tickets.

\begin{align*}4x\end{align*}

This is the first part of the variable expression because we don't know the price of the lift ticket.

Then there are four groups like Molly's family. We can multiply the variable expression by 4.

\begin{align*}4(4x)\end{align*}

Now we can simplify.

\begin{align*}16x\end{align*}

**This is the number of people who will need lift tickets.**

### Vocabulary

- Integer
- the set of whole numbers and their opposites.

- Product
- the answer in a multiplication problem

- Factors
- the numbers being multiplied

- Variable Expression
- a number phrase using numbers, operations and variables.

- Commutative Property of Multiplication
- states that the order that we multiply terms does not change the product.

- Associative Property of Multiplication
- states that changing the grouping of factors does not change the product.

### Guided Practice

Here is one for you to try on your own.

The temperature outside Fred's house is dropping at a rate of \begin{align*}2^\circ F\end{align*} each hour. Represent the total change in the temperature over the next 5 hours as an integer.

**Answer**

First, let's write an expressing to represent the situation.

\begin{align*}(-2)(5) = -10\end{align*}

**The temperature change was \begin{align*}-10^\circ\end{align*}.**

### Video Review

- This is a James Sousa video on multiplying integers. It is a supporting video for this Concept.

### Practice

Directions: Multiply each variable expression.

1. \begin{align*}(-7k)(-6)\end{align*}

2. \begin{align*}(-8)(3a)(b)\end{align*}

3. \begin{align*}-6a(b)(c)\end{align*}

4. \begin{align*}-8a(6b)\end{align*}

5. \begin{align*}(12y)(-3x)(-1)\end{align*}

6. \begin{align*}-8x(4)\end{align*}

7. \begin{align*}-a(5)(-4b)\end{align*}

8. \begin{align*}-2ab(12c)\end{align*}

9. \begin{align*}-12ab(12c)\end{align*}

10. \begin{align*}8x(12z)\end{align*}

11. \begin{align*}-2a(-14c)\end{align*}

12. \begin{align*}-12ab(11c)\end{align*}

13. \begin{align*}-22ab(-2c)\end{align*}

14. \begin{align*}18ab(12)\end{align*}

15. \begin{align*}-21a(-3b)\end{align*}