**Writing linear equations from word problems :**

We can use the information from the given real-world problem to write the linear equation that represents a given situation.

**Example 1 : **

The table shows the temperature of a fish tank during an experiment. Write the appropriate linear equation to find the temperature at any time.

**Solution : **

**Step 1 : **

Notice that the change in the temperature is the same for each increase of 1 hour in time. So, the relationship is linear.

**Step 2 : **

Let "x" stand for time and "y" stand for temperature.

Choose any two points in the form (x, y), from the table to find the slope :

For example, let us choose (0, 82) and (1, 80).

Use the slope formula.

m = (y₂ - y₁) / (x₂ - x₁)

Substitute (0, 82) for (x₁, y₁) and (1, 80) for (x₂, y₂).

m = (80 - 82) / (1 - 0)

m = -2 / 1

m = -2

**Step 3 : **

Find the y-intercept using the slope and any point from the table.

Slope-intercept form equation of a line :

y = mx + b

Plug m = -2, and (x, y) = (0, 82)

82 = -2(0) + b

82 = 0 + b

82 = b

**Step 4 : **

Now, plug m = -2 and b = 82 in slope-intercept form equation of a line.

y = mx + b

y = -2x + 82

**Example 2 : **

Elizabeth’s cell phone plan lets her choose how many minutes are included each month. The table shows the plan’s monthly cost y for a given number of included minutes x. Write an equation in slope-intercept form to represent the situation.

**Solution : **

**Step 1 : **

Notice that the change in cost is the same for each increase of 100 minutes. So, the relationship is linear.

**Step 2 : **

Choose any two points in the form (x, y), from the table to find the slope :

For example, let us choose (100, 14) and (200, 20).

Use the slope formula.

m = (y₂ - y₁) / (x₂ - x₁)

Substitute (100, 14) for (x₁, y₁) and (200, 20) for (x₂, y₂).

m = (20 - 14) / (200 - 100)

m = 6 / 100

m = 0.06

**Step 3 : **

Find the y-intercept using the slope and any point from the table.

Slope-intercept form equation of a line :

y = mx + b

Plug m = 0.06, and (x, y) = (100, 14)

14 = 0.06(100) + b

14 = 6 + b

8 = b

**Step 4 : **

Now, plug m = 0.06 and b = 8 in slope-intercept form equation of a line.

y = mx + b

y = 0.06x + 8

**Example 3 : **

The rent charged for space in an office building is a linear relationship related to the size of the space rented.At west main street office rentals, $750 rent charged for 600 square feet of space and $1150 rent charged for 900 square feet of space. Write an equation in slope-intercept form for the rent at West Main Street Office Rentals.

**Solution : **

**Step 1 :**

Identify the independent and dependent variables.

The independent variable (x) is the square footage of floor space.

The dependent variable (y) is the monthly rent.

**Step 2 :**

Write the information given in the problem as ordered pairs.

The rent for 600 square feet of floor space is $750 :

(600, 750)

The rent for 900 square feet of floor space is $1150 :

(900, 1150)

**Step 3 : **

Find the slope.

m = (y₂ - y₁) / (x₂ - x₁)

Substitute (600, 750) for (x₁, y₁) and (900, 1150) for (x₂, y₂).

m = (1150 - 750) / (900 - 600)

m = 400 / 300

m = 4/3

**Step 4 : **

Find the y-intercept.

Use the slope 4/3 and one of the ordered pairs (600, 750).

Slope-intercept form :

y = mx + b

Plug m = 4/3, x = 600 and y = 750.

750 = (4/3)(600) + b

750 = (4)(200) + b

750 = 800 + b

-50 = b

**Step 5 : **

Substitute the slope and y-intercept.

Slope-intercept form

y = mx + b

Plug m = 4/3 and b = -50

y = (4/3)x + (-50)

y = (4/3)x - 50

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