Suppose a coordinate plane is transposed over the map of a city, and Main Street has the equation

### Equations of Parallel Lines

Recall that **parallel lines** have the same slope.

Each of the graphs below have the same slope, which is 2. According to the definition, all these lines are parallel.

#### Let's determine if y=13x−4 and −3x+9y=18 are parallel:

The slope of the first line is

Any line parallel to this must also have a slope of

Find the slope of the second equation:

These two lines have the same slope so they are parallel.

#### Writing Equations of Parallel Lines

Sometimes, you will asked to write the equation of a line parallel to a given line that goes through a given point. In the following example, you will see how to do this.

#### Let's find the equation of the following lines:

- The line parallel to the line
y=6x−9 passing through (–1, 4).

Parallel lines have the same slope, so the slope will be 6. You have a point and the slope, so you can use point-slope form.

You could rewrite it in slope-intercept form:

- The line parallel to the line
y−5=2(x+3) passing through (1, 1).

First, we notice that this equation is in point-slope form, so let's use point-slope form to write this equation.

### Examples

#### Example 1

Earlier, you were told that a coordinate plane is transposed over the map of a city, and Main Street has the equation

Since Broad Street is parallel to Main Street, the line representing the two streets have the same slope. The slope of the equation for Main Street is 2 so the slope of the equation for Broad Street is also 2. Now, we can use the slope and the point (3, 14) to find the equation for Broad Street.

#### Example 2

Find the equation of the line parallel to the line

Since this is in standard form, we must first find the slope. For

Now that we have the slope, we can plug it in:

### Review

- Define parallel lines.

In 2-6, determine the slope of a line parallel to each line given.

y=−5x+7 2x+8y=9 x=8 y=−4.75 y−2=15(x+3)

In 7-10, find the line parallel to it through the given point.

y=−35x+2;(0,−2) 5x−2y=7;(2,−10) x=y;(2,3) x=−5;(−2,−3)

**Mixed Review**

- Graph the equation
2x−y=10 . - On a model boat, the stack is 8 inches high. The actual stack is 6 feet tall. How tall is the mast on the model if the actual mast is 40 feet tall?
- The amount of money charged for a classified advertisement is directly proportional to the length of the advertisement. If a 50-word advertisement costs $11.50, what is the cost of a 70-word advertisement?
- Simplify
112−−−√ . - Simplify
122−72−−−−−−−√ . - Is
3√−2√ rational, irrational, or neither? Explain your answer. - Solve for
s: 15s=6(s+32) .

### Review (Answers)

To see the Review answers, open this PDF file and look for section 5.7.