1.7: Applications Using Linear Equations

Module 1 Learning Objectives

1.7: Applications Using Linear Equations

  • Direct Translation – Translate to an algebraic equation and solve
  • Consecutive Integers – Use an algebraic equation to find consecutive integers, consecutive even integers, or consecutive odd integers when given the sum
  • Perimeter – Use an algebraic equation to find the dimensions of a rectangle when given the perimeter
  • Comparison Applications – Given information about how two unknowns are related, use an algebraic equation to solve for the unknowns

 

Define a Process for Problem Solving

The power of algebra is how it can help you model real situations in order to answer questions about them.

Here are some steps to translate problem situations into algebraic equations you can solve. Not every word problem fits perfectly into these steps, but they will help you get started.

  1. Read and understand the problem.
  2. Determine the constants and variables in the problem.
  3. Translate words into algebraic expressions and equations.
  4. Write an equation to represent the problem.
  5. Solve the equation.
  6. Check and interpret your answer. Sometimes writing a sentence helps.

Direct Translation

Word problems can be tricky. Often it takes a bit of practice to convert an English sentence into a mathematical sentence, which is one of the first steps to solving word problems. In the table below, words or phrases commonly associated with mathematical operators are categorized. Word problems often contain these or similar words, so it’s good to see what mathematical operators are associated with them.

Addition [latex]+[/latex] Subtraction [latex]-[/latex] Multiplication [latex]\times[/latex] Division[latex]\div[/latex] Variable ? Equals [latex]=[/latex]
More than Less than Double Ratio A number Is
Together In the past Product Quotient Often, a value for which no information is given. The same as
Sum Slower than Times Per After how many hours?
Total The remainder of Of How much will it cost?
In the future Difference
Faster than

Some examples follow:

  • [latex]x\text{ is }5[/latex]  becomes [latex]x=5[/latex]
  • Three more than a number becomes [latex]x+3[/latex]
  • Four less than a number becomes [latex]x-4[/latex]
  • Double the cost becomes [latex]2\cdot\text{ cost }[/latex]
  • Groceries and gas together for the week cost $250 means [latex]\text{ groceries }+\text{ gas }=250[/latex]
  • The difference of 9 and a number becomes [latex]9-x[/latex]. Notice how 9 is first in the sentence and the expression

Let’s practice translating a few more English phrases into algebraic expressions.

Example 1

Translate each phrase in the table into algebraic expressions:

 some number  the sum of the number and 3  twice the sum of the number and 3
 a length  double the length  double the length, decreased by 6
 a cost  the difference of the cost and 20  2 times the difference of the cost and 20
 some quantity  the difference of 5 and the quantity   the difference of 5 and the quantity, divided by 2
 an amount of time  triple the amount of time  triple the amount of time, increased by 5
 a distance  the sum of [latex]-4[/latex] and the distance  the sum of [latex]-4[/latex] and the twice the distance

In this example video, we show how to translate more words into mathematical expressions.

 

Example 2

Twenty-eight less than five times a certain number is 232. What is the number?

In the video that follows, we show another example of how to translate a sentence into a mathematical expression using a problem solving method.

In the last direct translation example, we see what language would require us to utilize parentheses.

Example 3

Three times the difference of a number and 4 is 18.  Find the number.

 

Consecutive Integers

Consecutive Integers

Another type of number problem involves consecutive numbers. Consecutive numbers are numbers that come one after the other, such as 3, 4, 5. If we are looking for several consecutive numbers it is important to first identify what they look like with variables before we set up the equation.

For example, let’s say I want to know the next consecutive integer after 4. In mathematical terms, we would add 1 to 4 to get 5. We can generalize this idea as follows: the consecutive integer of any number, x, is [latex]x+1[/latex]. If we continue this pattern we can define any number of consecutive integers from any starting point. The following table shows how to describe four consecutive integers using algebraic notation.

First [latex]x[/latex]
Second [latex]x+1[/latex]
Third [latex]x+2[/latex]
Fourth  [latex]x+3[/latex]

We apply the idea of consecutive integers to solving a word problem in the following example.

Example 4

The sum of three consecutive integers is 93. What are the integers?

In the following video we show another example of a consecutive integer problem.

Consecutive Even or Odd Integers

The following are examples of consecutive odd integers:

7 and 9 are two consecutive odd integers. -19 and -17 are a different set of two consecutive odd integers.

Notice that 7+2 = 9 and -19+2 = -17.

In order to go from one odd integer to the next consecutive odd integer you need to add 2.

The following are examples of consecutive even integers:

10 and 12 are two consecutive even integers.  -44 and -42 are a different set of two consecutive even integers

Notice that 10+2 = 12 and -44+2 = -42.

In order to go from one even integer to the next consecutive even integer you need to add 2.

Based on these examples, we can use the following labels when we are asked to find the following:

Consecutive Integers:             [latex]x, x+1, x+2, ...[/latex]

Consecutive Odd Integers:     [latex]x, x+2, x+4, ...[/latex]

Consecutive Even Integers:    [latex]x, x+2, x+4, ...[/latex]

 

Example 5

The sum of two consecutive even integers is -74. Find the integers.

Example 6

The sum of three consecutive odd integers is -15. List the integers from smallest to largest.

 

Perimeter

Perimeter is the distance around an object. For example, consider a rectangle with a length of 8 and a width of 3. There are two lengths and two widths in a rectangle (opposite sides), so we add [latex]8+8+3+3=22[/latex]. Since there are two lengths and two widths in a rectangle, you may find the perimeter of a rectangle using the formula [latex]{P}=2\left({L}\right)+2\left({W}\right)[/latex] where

L = Length

W = Width

In the following example, we will use the problem-solving method we developed to find an unknown width using the formula for the perimeter of a rectangle. By substituting the dimensions we know into the formula, we will be able to isolate the unknown width and find our solution.

Example 7

You want to make another garden box the same size as the one you already have. You write down the dimensions of the box and go to the lumber store to buy some boards. When you get there you realize you didn’t write down the width dimension—only the perimeter and length. You want the exact dimensions so you can have the store cut the lumber for you.

Here is what you have written down:

Perimeter = 16.4 feet
Length = 4.7 feet

Can you find the dimensions you need to have your boards cut at the lumber store? If so, how many boards do you need and what lengths should they be?

 

The perimeter of any shape can be found by adding the lengths of all the sides.  The first step in each of the following examples is to define the variable and which side length it represents.  Then label each side of the figure in terms of the same variable.  Once all the sides of the figure are labeled, the sum of the lengths of the sides should equal the given perimeter.

Example 8

A rectangular room is 7 meters longer than it is wide, and its perimeter is 62 meters.  Find the dimensions of the room.

Example 9

One side of a triangle is twice as long as the shortest side, and the third side is four times as long as the shortest side.  The perimeter is 63 feet.  Find the dimensions of the triangle.

Comparison Applications

The first step in these problems is to define the variable and which unknown in the problem it represents.  Then define the other unknown(s) in the problem in terms of that same variable.  Next, read the problem carefully to create an equation.  Solve the equation and be sure to label the answers.

Example 10

A sofa and a loveseat together cost $630. The cost of the sofa is twice the cost of the loveseat.  How much do they each cost?

Example 11

A bag is filled with green and blue marbles. There are 111 marbles in the bag.  If there are 17 more green marbles than blue marbles, find the number of green marbles and the number of blue marbles in the bag.

Example 12

A 12-foot board is cut into two pieces.  One piece is 4 feet shorter than the other piece. How long are the pieces?