Lines and also linear equations

Graphs of lines

Geometry taught us that precisely one line crosses through any two points. We deserve to use this fact in algebra together well. When drawing the graph the a line, we only require two points, and then use a right edge to attach them. Remember, though, that lines are infinitely long: they do not start and stop in ~ the two points we provided to attract them.

Lines deserve to be expressed algebraically together an equation that relates the $y$-values to the $x$-values. We deserve to use the same fact that we used previously that 2 points are contained in exactly one line. With only two points, we can determine the equation of a line. Prior to we do this, let"s discuss some an extremely important characteristics of lines: slope, $y$-intercept, and also $x$-intercept.


*

slope

Think of the steep of a line as its "steepness": how conveniently it rises or drops from left to right. This worth is shown in the graph above as $fracDelta yDelta x$, i m sorry specifies how much the line rises or drops (change in $y$) together we relocate from left to appropriate (change in $x$). That is crucial to relate steep or steepness come the price of vertical change per horizontal change. A popular instance is that of speed, which measures the change in distance per adjust in time. Wherein a line deserve to represent the distance traveled at various points in time, the steep of the heat represents the speed. A steep heat represents high speed, conversely, very tiny steepness represents a lot slower rate of travel, or short speed. This is depicted in the graph below.


*

Speed graph


The upright axis represents distance, and also the horizontal axis represents time. The red line is steeper 보다 the blue and also green lines. Notification the distance traveled after one hour ~ above the red heat is around 5 miles. The is much higher than the street traveled on the blue or environment-friendly lines ~ one hour - about $1$ mile and also $frac15$, respectively. The steeper the line, the greater the street traveled every unit the time. In various other words, steepness or slope to represent speed. The red present is the fastest, with the greatest slope, and also the environment-friendly line is the slowest, with the smallest slope.

Slope deserve to be classified in 4 ways: positive, negative, zero, and undefined slope. Hopeful slope means that as we relocate from left to ideal on the graph, the line rises. An unfavorable slope way that together we relocate from left to right on the graph, the line falls. Zero slope way that the line is horizontal: it no rises nor drops as we relocate from left come right. Vertical lines are claimed to have "undefined slope," together their slope appears to be part infinitely large, undefined value. Check out the graphs listed below that present each the the 4 slope types.


positive slope: negative slope: Zero slope (Horizontal): undefined slope (Vertical):
$fracDelta yDelta x gt 0$ $fracDelta yDelta x lt 0$ $Delta y = 0$, $Delta x eq 0$, for this reason $fracDelta yDelta x = 0$ $Delta x = 0$, so $fracDelta yDelta x$ is unknown
*
*
*
*

Investigate the habits of a line by adjusting the steep via the "$m$-slider".

Watch this video on steep for an ext insight right into the concept.


$y$-Intercept

The $y$-intercept that a heat is the suggest where the line crosses the $y$-axis. Note that this happens when $x = 0$. What space the $y$-intercepts the the currently in the graphs above?

that looks like the $y$-intercepts room $(0, 1)$, $(0, 0)$, and also $(0, 1)$ because that the first three graphs. Over there is no $y$-intercept ~ above the fourth graph - the line never crosses the $y$-axis. Investigate the behavior of a heat by adjusting the $y$-intercept via the "$b$-slider".

$x$-Intercept

The $x$-intercept is a similar concept together $y$-intercept: that is the allude where the line crosses the $x$-axis. This happens as soon as $y = 0$. The $x$-intercept is not offered as often as $y$-intercept, together we will certainly see when determing the equation the a line. What are the $x$-intercepts the the currently in the graphs above?

the looks prefer the $x$-intercepts space $(-frac12, 0)$ and $(0, 0)$ for the first two graphs. Over there is no $x$-intercept on the third graph. The fourth graph has an $x$-intercept at $(-1, 0)$.

Equations of currently

In order to write an equation that a line, us usually have to determine the steep of the heat first.

Calculating slope

Algebraically, steep is calculated as the proportion of the adjust in the $y$ value to the adjust in the $x$ worth between any kind of two clues on the line. If we have actually two points, $(x_1, y_1)$ and also $(x_2, y_2)$, steep is express as:

$$box extslope = m = fracDelta yDelta x = fracy_2 - y_1x_2 - x_1.$$

note that we usage the letter $m$ to denote slope. A line the is an extremely steep has actually $m$ values through very huge magnitude, whereas together line that is not steep has $m$ values through very small magnitude. Because that example, slopes of $100$ and also $-1,000$ have much bigger magnitude than slopes of $-0.1$ or $1$.

Example:

uncover the slope of the line the passes through points $(-2, 1)$ and $(5, 8)$.

making use of the formula because that slope, and also letting suggest $(x_1, y_1) = (-2, 1)$ and point $(x_2, y_2) = (5, 8)$, $$eginalign* m &= fracDelta yDelta x = fracy_2 - y_1x_2 - x_1\<1ex> &= frac8 - 15 - (-2)\<1ex> &= frac75 + 2\<1ex> &= frac77\<1ex> &= 1 endalign*$$

note that we chose allude $(-2, 1)$ as $(x_1, y_1)$ and suggest $(5, 8)$ together $(x_2, y_2)$. This to be by choice, together we might have let allude $(5, 8)$ it is in $(x_1, y_1)$ and point $(-2, 1)$ it is in $(x_1, y_1)$. Exactly how does that affect the calculate of slope?

$$eginalign* m &= fracDelta yDelta x = fracy_2 - y_1x_2 - x_1\<1ex> &= frac1 - 8-2 - 5\<1ex> &= frac-7-7\<1ex> &= 1 endalign*$$

We watch the steep is the very same either method we pick the first and 2nd points. We deserve to now conclude the the slope of the line the passes through points $(-2, 1)$ and also $(5, 8)$ is $1$.

Watch this video for an ext examples ~ above calculating slope.

currently that we know what slope and $y$-intercepts are, we have the right to determine the equation of a heat given any type of two point out on the line. There room two primary ways to create the equation the a line: point-slope form and slope-intercept form. We will very first look in ~ point-slope form.

Point-Slope kind

The point-slope type of one equation the passes with the allude $(x_1, y_1)$ v slope $m$ is the following:

$$box extPoint-Slope form: y - y_1 = m(x - x_1).$$ Example:

What is the equation of the line has slope $m = 2$ and also passes through the allude $(5, 4)$ in point-slope form?

utilizing the formula for the point-slope type of the equation the the line, we deserve to just substitute the steep and allude coordinate worths directly. In other words, $m = 2$ and $(x_1, y_2) = (5, 4)$. So, the equation of the heat is $$y - 4 = 2(x - 5).$$

Example:

offered two points, $(-3, -5)$ and also $(2, 5)$, create the point-slope equation the the line that passes with them.

First, us calculate the slope: $$eginalign* m &= fracy_2 - y_1x_2 - x_1\<1ex> &= frac5 - (-5)2 - (-3)\<1ex> &= frac105\<1ex> &= 2 endalign*$$

Graphically, we deserve to verify the slope by looking in ~ the readjust in $y$-values matches the readjust in $x$-values between the 2 points:


*

Graph of line passing through $(2, 5)$ and $(-3, -5)$.

You are watching: Equation of a vertical line in standard form


We can now use one of the points along with the steep to create the equation that the line: $$eginalign* y - y_1 &= m(x - x_1) \ y - 5 &= 2(x - 2) quadcheckmark endalign*$$

us could additionally have supplied the other point to write the equation the the line: $$eginalign* y - y_1 &= m(x - x_1) \ y - (-5) &= 2(x - (-3)) \ y + 5 &= 2(x + 3) quadcheckmark endalign*$$

however wait! Those two equations look different. How have the right to they both describe the same line? If we simplify the equations, we watch that castle are certainly the same. Let"s do just that: $$eginalign* y - 5 &= 2(x - 2) \ y - 5 &= 2x - 4 \ y - 5 + 5 &= 2x - 4 + 5 \ y &= 2x + 1 quadcheckmark endalign*$$ $$eginalign* y + 5 &= 2(x + 3) \ y + 5 &= 2x + 6 \ y + 5 - 5 &= 2x + 6 - 5 \ y &= 2x + 1 quadcheckmark endalign*$$

So, using either allude to compose the point-slope form of the equation results in the very same "simplified" equation. We will see following that this simplified equation is another important type of straight equations.

Slope-Intercept form

Another method to express the equation of a heat is slope-intercept form.

$$box extSlope-Intercept form: y = mx + b.$$

In this equation, $m$ again is the steep of the line, and also $(0, b)$ is the $y$-intercept. Choose point-slope form, every we need are 2 points in stimulate to write the equation the passes with them in slope-intercept form.

Constants vs. Variables

the is important to keep in mind that in the equation for slope-intercept form, the letter $a$ and also $b$ are constant values, together opposed come the letters $x$ and also $y$, which space variables. Remember, constants stand for a "fixed" number - it does no change. A variable can be one of many values - it have the right to change. A offered line includes many points, each of which has a unique $x$ and $y$ value, however that line only has one slope-intercept equation through one value each for $m$ and $b$.


Example:

offered the same two points above, $(-3, -5)$ and $(2, 5)$, compose the slope-intercept form of the equation that the line that passes v them.

We already calculated the slope, $m$, over to be $2$. We can then use among the point out to fix for $b$. Making use of $(2, 5)$, $$eginalign* y &= 2x + b \ 5 &= 2(2) + b \ 5 &= 4 + b \ 1 &= b. endalign*$$ So, the equation of the heat in slope-intercept type is, $$y = 2x + 1.$$ The $y$-intercept that the line is $(0, b) = (0, 1)$. Look at the graph above to verify this is the $y$-intercept. In ~ what suggest does the line cross the $y$-axis?

At an initial glance, it seems the point-slope and also slope-intercept equations of the line space different, but they really do describe the very same line. We have the right to verify this through "simplifying" the point-slope type as such: $$eginalign* y - 5 &= 2(x - 2) \ y - 5 &= 2x - 4 \ y - 5 + 5 &= 2x - 4 + 5 \ y &= 2x + 1 \ endalign*$$

Watch this video for much more examples on composing equations of present in slope-intercept form.

Horizontal and Vertical lines

currently that we have the right to write equations the lines, we require to consider two special instances of lines: horizontal and also vertical. Us claimed above that horizontal lines have slope $m = 0$, and that upright lines have undefined slope. How can we usage this to determine the equations of horizontal and vertical lines?

upright lines
Facts around vertical present If two points have the very same $x$-coordinates, only a upright line have the right to pass v both points. Each suggest on a vertical line has the same $x$-coordinate. If two points have actually the very same $x$-coordinate, $c$, the equation of the heat is $x = c$. The $x$-intercept of a vertical heat $x = c$ is the point $(c, 0)$. other than for the heat $x = 0$, vertical lines do not have a $y$-intercept.
Example:

think about two points, $(2, 0)$ and $(2, 1)$. What is the equation that the line the passes v them?


*

Graph of heat passing with points $(2, 0)$ and also $(2, 1)$


First, keep in mind that the $x$-coordinate is the very same for both points. In fact, if us plot any allude from the line, we deserve to see that the $x$-coordinate will be $2$. We know that only a upright line have the right to pass through the points, therefore the equation of the line must be $x = 2$.

But, how can we verify this algebraically? very first off, what is the slope? we calculate slope as $$eginalign* m &= frac1 - 02 - 2 \<1ex> &= frac10 \<1ex> &= extundefined endalign*$$ In this case, the slope worth is undefined, which renders it a upright line.

Slope-intercept and also point-slope creates

at this point, you could ask, "how have the right to I write the equation that a vertical line in slope-intercept or point-slope form?" The answer is that you really have the right to only compose the equation of a vertical heat one way. For vertical lines, $x$ is the same, or constant, for all values of $y$. Due to the fact that $y$ can be any kind of number for vertical lines, the change $y$ does not show up in the equation that a upright line.

Horizontal currently
Facts around horizontal present If 2 points have the very same $y$-coordinates, only a horizontal line deserve to pass with both points. Each suggest on a horizontal line has the same $y$-coordinate. If 2 points have the very same $y$-coordinate, $b$, the equation that the heat is $y = b$. The $y$-intercept the a horizontal heat $y = b$ is the point $(0, b)$. other than for the line $y = 0$, horizontal lines do not have actually an $x$-intercept.
Example:

take into consideration two points, $(3, 4)$ and $(0, 4)$. What is the equation that the line the passes v them?


*

Graph of heat passing through points $(3, 4)$ and also $(0, 4)$


First, keep in mind that the $y$-coordinate is the very same for both points. In fact, if we plot any suggest on the line, we can see the the $y$-coordinate is $4$. We understand that only a horizontal line can pass v the points, therefore the equation of the line should be $y = 4$.

How can we verify this algebraically? First, calculate the slope: $$eginalign* m &= frac4 - 40 - 3 \<1ex> &= frac0-3 \<1ex> &= 0 endalign*$$ Then, making use of slope-intercept form, we can substitute $0$ because that $m$, and also solve because that $y$: $$eginalign* y &= (0)x + b \<1ex> &= b endalign*$$ This tells united state that every point on the line has $y$-coordinate $b.$ since we know two clues on the line have $y$-coordinate $4$, $b$ have to be $4$, and so the equation that the line is $y = 4$.

Slope-intercept and also Point-slope develops

similar to vertical lines, the equation that a horizontal line deserve to only be written one way. For horizontal lines, $y$ is the very same for all worths of $x$. Because $x$ can be any number because that horizontal lines, the change $x$ walk not show up in the equation the a horizontal line.

Parallel and Perpendicular currently

now that we know exactly how to characterize lines by their slope, we have the right to identify if 2 lines space parallel or perpendicular by their slopes.

Parallel currently

In geometry, we space told the two distinct lines that execute not intersect are parallel. Looking in ~ the graph below, there space two lines the seem to never to intersect. What have the right to we say around their slopes?


*

It shows up that the lines above have the exact same slope, and that is correct. Non-vertical parallel lines have actually the same slope. Any type of two vertical lines, however, are also parallel. That is essential to note that vertical lines have actually undefined slope.

Perpendicular present

We recognize from geometry the perpendicular lines form an edge of $90^circ$. The blue and also red currently in the graph below are perpendicular. What execute we an alert about their slopes?


also though this is one particular example, the relationship in between the slopes applies to all perpendicular lines. Skipping the indications for now, notification the vertical adjust in the blue line amounts to the horizontal change in the red line. Likewise, the the vertical change in the red line amounts to the horizontal adjust in the blue line. So, then, what are the slopes that these 2 lines? $$ extslope of blue line = frac-21 = -2$$ $$ extslope of red line = frac12$$

The other truth to notification is that the indications of the slopes that the lines are not the same. The blue line has a negative slope and the red line has actually a optimistic slope. If us multiply the slopes, us get, $$-2 imes frac12 = -1.$$ This station and an adverse relationship in between slopes is true for all perpendicular lines, other than horizontal and vertical lines.

below is another example of two perpendicular lines:


$$ extslope that blue line = frac-23$$ $$ extslope of red line = frac32$$ $$ extProduct the slopes = frac-23 cdot frac32 = -1$$ Again, we see that the slopes of 2 perpendicular lines are an adverse reciprocals, and therefore, your product is $-1$. Recall that the reciprocal of a number is $1$ separated by the number. Let"s verify this with the examples above: The an adverse reciprocal that $-2$ is $-frac1-2 = frac12 checkmark$. The negative reciprocal of $frac12$ is $-frac1frac12 = -2 checkmark$. The negative reciprocal the $-frac23$ is $-frac1-frac23 = frac32 checkmark$. The an adverse reciprocal that $frac32$ is $-frac1frac32 = -frac23 checkmark$.


2 lines space perpendicular if one of the following is true: The product of their slopes is $-1$. One line is vertical and the various other is horizontal.

Exercises

Calculate the slope of the heat passing with the given points.

1. $(2, 1)$ and $(6, 9)$ 2. $(-4, -2)$ and also $(2, -3)$ 3. $(3, 0)$ and also $(6, 2)$
4. $(0, 9)$ and also $(4, 7)$ 5. $(-2, frac12)$ and $(-5, frac12)$ 6. $(-5, -1)$ and $(2, 3)$
7. $(-10, 3)$ and also $(-10, 4)$ 8. $(-6, -4)$ and also $(6, 5)$ 9. $(5, -2)$ and $(-4, -2)$

Find the steep of each of the adhering to lines.

10. $y - 2 = frac12(x - 2)$ 11. $y + 1 = x - 4$ 12. $y - frac23 = 4(x + 7)$
13. $y = -(x + 2)$ 14. $2x + 3y = 6$ 15. $y = -2x$
16. $y = x$ 17. $y = 4$ 18. $x = -2$
19. $x = 0$ 20. $y = -1$ 21. $y = 0$

Write the point-slope kind of the equation of the line with the offered slope and also containing the given point.

22. $m = 6$; $(2, 7)$ 23. $m = frac35$; $(9, 2)$ 24. $m = -5$; $(6, 2)$
25. $m = -2$; $(-4, -1)$ 26. $m = 1$; $(-2, -8)$ 27. $m = -1$; $(-3, 6)$
28. $m = frac43$; $(7, -1)$ 29. $m = frac72$; $(-3, 4)$ 30. $m = -1$; $(-1, -1)$

Write the point-slope kind of the equation that the line passing with the provided pair that points.

31. $(1, 5)$ and also $(4, 2)$ 32. $(3, 7)$ and also $(4, 8)$ 33. $(-3, 1)$ and $(3, 5)$
34. $(-2, 3)$ and also $(3, 5)$ 35. $(5, 0)$ and $(0, -2)$ 36. $(-2, 0)$ and $(0, 3)$
37. $(0, 0)$ and $(-1, 1)$ 38. $(1, 1)$ and $(3, 1)$ 39. $(3, 2)$ and also $(3, -2)$

Exercises 40-48: compose the slope-intercept kind of the equation the the line through the given slope and also containing the given suggest in exercises 22-30.

See more: L Is Running 4 Miles A Day Good, Run 5 Miles A Day

Exercises 49-57: compose the slope-intercept kind of the equation that the heat passing through the provided pair of clues in exercises 31-39.