103 Write the equation of a line parallel or perpendicular to a given line

Writing Equations of Parallel Lines

Suppose for example, we are given the following equation.

We know that the slope of the line formed by the function is 3. We also know that the y-intercept is (0, 1). Any other line with a slope of 3 will be parallel to f(x). So the lines formed by all of the following functions will be parallel to f(x).

Suppose then we want to write the equation of a line that is parallel to f and passes through the point (1, 7). We already know that the slope is 3. We just need to determine which value for b will give the correct line. We can begin with the point-slope form of an equation for a line, and then rewrite it in the slope-intercept form.

So [latex]g\left(x\right)=3x+4\\[/latex] is parallel to [latex]f\left(x\right)=3x+1\\[/latex] and passes through the point (1, 7).

Example 9: Finding a Line Parallel to a Given Line

Find a line parallel to the graph of [latex]f\left(x\right)=3x+6\\[/latex] that passes through the point (3, 0).

Solutions

The slope of the given line is 3. If we choose the slope-intercept form, we can substitute m = 3, x = 3, and f(x) = 0 into the slope-intercept form to find the y-intercept.

[latex]\begin{cases}g\left(x\right)=3x+b\hfill \\ \text{ }0=3\left(3\right)+b\hfill \\ \text{ }b=-9\hfill \end{cases}\\[/latex]

The line parallel to f(x) that passes through (3, 0) is [latex]g\left(x\right)=3x - 9\\[/latex].

Analysis of the Solution

We can confirm that the two lines are parallel by graphing them. Figure 22 shows that the two lines will never intersect.

Writing Equations of Perpendicular Lines

We can use a very similar process to write the equation for a line perpendicular to a given line. Instead of using the same slope, however, we use the negative reciprocal of the given slope. Suppose we are given the following function:

The slope of the line is 2, and its negative reciprocal is [latex]-\frac{1}{2}\\[/latex]. Any function with a slope of [latex]-\frac{1}{2}\\[/latex] will be perpendicular to f(x). So the lines formed by all of the following functions will be perpendicular to f(x).

As before, we can narrow down our choices for a particular perpendicular line if we know that it passes through a given point. Suppose then we want to write the equation of a line that is perpendicular to f(x) and passes through the point (4, 0). We already know that the slope is [latex]-\frac{1}{2}\\[/latex]. Now we can use the point to find the y-intercept by substituting the given values into the slope-intercept form of a line and solving for b.

The equation for the function with a slope of [latex]-\frac{1}{2}\\[/latex] and a y-intercept of 2 is

So [latex]g\left(x\right)=-\frac{1}{2}x+2\\[/latex] is perpendicular to [latex]f\left(x\right)=2x+4\\[/latex] and passes through the point (4, 0). Be aware that perpendicular lines may not look obviously perpendicular on a graphing calculator unless we use the square zoom feature.

Example 10: Finding the Equation of a Perpendicular Line

Find the equation of a line perpendicular to [latex]f\left(x\right)=3x+3\\[/latex] that passes through the point (3, 0).

Solution

The original line has slope m = 3, so the slope of the perpendicular line will be its negative reciprocal, or [latex]-\frac{1}{3}\\[/latex]. Using this slope and the given point, we can find the equation for the line.

[latex]\begin{cases}g\left(x\right)=-\frac{1}{3}x+b\hfill \\ \text{ }0=-\frac{1}{3}\left(3\right)+b\hfill \\ \text{ }1=b\hfill \\ \text{ }b=1\hfill \end{cases}\\[/latex]

The line perpendicular to f(x) that passes through (3, 0) is [latex]g\left(x\right)=-\frac{1}{3}x+1\\[/latex].

Analysis of the Solution

A graph of the two lines is shown in Figure 23.