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### Course: Algebra 1 > Unit 4

Lesson 2: Slope- Intro to slope
- Positive & negative slope
- Worked example: slope from graph
- Slope from graph
- Graphing a line given point and slope
- Graphing from slope
- Calculating slope from tables
- Slope in a table
- Worked example: slope from two points
- Slope from two points
- Slope review

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# Worked example: slope from two points

Let's find the slope of the line that goes through the ordered pairs (4,2) and (-3, 16). The slope, or steepness, of a line is found by dividing the vertical change (rise) by the horizontal change (run). The formula is slope =(y₂ - y₁)/(x₂ - x₁), where (x₁, y₁) and (x₂, y₂) are the coordinates of two points on the line. Created by Sal Khan and Monterey Institute for Technology and Education.

## Want to join the conversation?

- Do I HAVE to use this method, or is it okay to just use the other one in the previous video, "Graphical Slope of a Line"? Is there really any difference?(282 votes)
- The method of graphing the line and then measuring the slope of the graph isn't a very good method when you are dealing with messy numbers (fractions, irrational numbers etc.) Also, the method used in this video will work even if you don't have graph paper.(290 votes)

- How do you find the slope of a curved line?? (not linear)(67 votes)
- There is no such thing as the "slope of a curve" per se; what you have to find is the slope of the line that hugs the curve closely at a given point, called the tangent line at that point. You can find this by taking the derivative of the equation of the curve and then plugging in the x value of that point. That's the very beginning of calculus; you can watch Sal's videos on taking derivatives in the Calculus section. ^_^(155 votes)

- but if you do like Sal did - simplifying 14/-2 to -2

i can't figure out the slope!!(34 votes)- Remember that -2 can be written like -2/1. :) Since -2 divided by 1 is just -2.

Do you see the rise over run now? -2 is the "rise", while 1 is the "run".

14/-7 is a proportional fraction to -2/1. You can simplify the fraction by dividing both numbers by 7.

14 divided by 7 is 2, and -7 divided 7 is -1. So, 14/-7 is equal to -2/1.

Since they are proportional fractions, their slope is actually the same too! :)

Let's try with Sal's examples, the starting point at (-3,16) to the ending point (4,2). With the rise over run -2/1 (which is the same as 14/-7 remember? Just simplified), we go 2 down and 1 right. Our starting point is now at (-2,14).

Let's go 2 down and 1 right again. Our point is now (-1,12).

Again. (0,10).

Again. :P (1,8)

Almost there. (2,6)

You see it? (3,4)

Well what do you know, now we are at (4,2)! The ending point we were aiming for.

Now, if you drew a line through all the coordinates we just made, it would look just like the line Sal drew. In fact, his line went through all the points that we made!

Try it yourself if you really care :P. Also, I'm sure you meant -7 right XP?(81 votes)

- What happens if you already have the line, and need the ordered pairs?(16 votes)
- Watch the "Slope of a line" video. (The one before this one). The whole video is about finding slope WITHOUT needing the ordered pairs.(14 votes)

- How do you draw a slope on a graph when the only number you are given is the slope? Say the problem says "show the slope of 2." How would you know how to draw the line without any coordinates?(12 votes)
- Janine,

You can drawn it anywhere on the graph. Just choose any first point. Then go up two and right 1 for the second point and draw a line through the two points. Your line will have a slope of 2.(15 votes)

- 1. Do you have to draw an arrow head at each end of the line (see6:00)? In other words, is this a "line" in the
**geometrical**sense? Or is it a line segment? Or perhaps a ray that goes from (-3,16) to (4,2)?

I am thinking about the way he describes it as y decreases when x increases. I am also thinking about what it means that x is independent and y is dependent variable. Can you really go in either direction on the x in this situation?

2. How do you explicitly tell on a Cartesian plane that you are only investigating the change in x in the positive direction? Can you draw the x axis as a ray with only an arrow head at the right side and a point on the left side?(11 votes)- 1. the question is ''find the slope of the
*line*''

so, yes it is a line in the geometrical sense.. and the line passes through the 2 points given

the equation of a line is y= mx +c

where y is dependent on x, m and c are constants and x is independent

and yes, you can go in both directions.. recall that a line goes on to infinity.. so, in principle there is no defined start point (therefore no start x-coordinate)

if x 'starts' from negative infinity and starts increasing, it is understood that it goes in both directions (positive and negative)

2. I'm not sure how to explain this one.. have you already watched the other videos?(11 votes)

- How can you explain it more simply because I am still struggling to understand?(11 votes)
- Hey there snoman! When finding slope of any 2 ordered pairs, for instance lets just use (2,9) and (19,10), a simple and quick method you can use is
*y2-y1*over*x2-x1*. Then just make sure you divide the y over the x in the end. Let me further explain using my example in 3 straightforward steps:**Step 1**: You take the ordered pairs*(2,9)*and*(19,10)*and take out the*y2*and*y1*numbers.**That would be 10 and 9**. Then take out the*x2*and*x1*numbers.**That should be 19 and 2**.**Step 2**: Next, you should minus the*y2*and*y1*numbers from each other.**Your answer should be 1**. After you do that, repeat the same process with the numbers of*x2*and*x1*and subtract both.**Your answer should be 17**.**Step 3**: Final one! This is very important so you get your slope! Now, we have to use the method of*rise/run*or y change over x change to get the final answer. Divide your y number (1) over your x number (17).**Your final answer should be 1/17**.(8 votes)

- how do you know if your answer is positive or negative?(1 vote)
- If the line goes up and to the right, then the slope is positive.

If the line goes down and to the right, then the slope is negative.

If the line is horizontal, then the slope is zero.

If the line is vertical, then the slope is undefined.

Have a blessed, wonderful day!(23 votes)

- in the (slope from two points) exercise I'm having trouble defining what is y and what is x in a pair

is it y,x or x,y(3 votes)- In a ordered pair it goes (x,y). For example lets say we have the pair (8,5). In this case our x, or run is 8, and our y, or rise, is 5. Hope this helped!(7 votes)

- This probably doesn't matter to math, but how is Sal so good at drawing with his computer?(5 votes)

## Video transcript

Find the slope of
the line that goes through the ordered pairs
4 comma 2 and negative 3 comma 16. So just as a reminder, slope
is defined as rise over run. Or, you could view that rise
is just change in y and run is just change in x. The triangles here,
that's the delta symbol. It literally means "change in." Or another way, and you
might see this formula, and it tends to be
really complicated. But just remember it's just
these two things over here. Sometimes, slope will be
specified with the variable m. And they'll say that
m is the same thing-- and this is really the
same thing as change in y. They'll write y2 minus
y1 over x2 minus x1. And this notation tends
to be kind of complicated, but all this means
is, is you take the y-value of your endpoint
and subtract from it the y-value of your
starting point. That will essentially
give you your change in y. And it says take the
x-value of your endpoint and subtract from that the
x-value of your starting point. And that'll give
you change in x. So whatever of
these work for you, let's actually figure out
the slope of the line that goes through these two points. So we're starting
at-- and actually, we could do it both ways. We could start at this
point and go to that point and calculate the slope or
we could start at this point and go to that point
and calculate the slope. So let's do it both ways. So let's say that our starting
point is the point 4 comma 2. And let's say that our endpoint
is negative 3 comma 16. So what is the change
in x over here? What is the change in
x in this scenario? So we're going from
4 to negative 3. If something goes
from 4 to negative 3, what was it's change? You have to go
down 4 to get to 0, and then you have to go down
another 3 to get to negative 3. So our change in x
here is negative 7. Actually, let me
write it this way. Our change in x is equal
to negative 3 minus 4, which is equal to negative 7. If I'm going from 4 to
negative 3, I went down by 7. Our change in x is negative 7. Let's do the same thing
for the change in y. And notice, I implicitly
use this formula over here. Our change in x was this value,
our endpoint, our end x-value minus our starting x-value. Let's do the same thing
for our change in y. Our change in y. If we're starting at
2 and we go to 16, that means we moved up 14. Or another way you
could say it, you could take your ending
y-value and subtract from that your starting y-value
and you get 14. So what is the slope over here? Well, the slope is just
change in y over change in x. So the slope over
here is change in y over change in x, which
is-- our change in y is 14. And our change in
x is negative 7. And then if we want to simplify
this, 14 divided by negative 7 is negative 2. Now, what I want
to show you is, is that we could have done
it the other way around. We could have made
this the starting point and this the endpoint. And what we would have
gotten is the negative values of each of these, but then
they would've canceled out and we would still
get negative 2. Let's try it out. So let's say that our start
point was negative 3 comma 16. And let's say that our
endpoint is the 4 comma 2. 4 comma 2. So in this situation,
what is our change in x? Our change in x. If I start at negative
3 and I go to 4, that means I went up 7. Or if you want to
just calculate that, you would do 4 minus negative 3. 4 minus negative 3. But needless to say,
we just went up 7. And what is our change in y? Our change in y over here,
or we could say our rise. If we start at 16 and we end at
2, that means we went down 14. Or you could just say 2
minus 16 is negative 14. We went down by 14. This was our run. So if you say rise
over run, which is the same thing as change
in y over change in x, our rise is negative 14
and our run here is 7. So notice, these are
just the negatives of these values from
when we swapped them. So once again, this is
equal to negative 2. And let's just visualize this. Let me do a quick
graph here just to show you what a downward
slope would look like. So let me draw our two points. So this is my x-axis. That is my y-axis. So this point over
here, 4 comma 2. So let me graph it. So we're going to go
all the way up to 16. So let me save some space here. So we have 1, 2, 3, 4. It's 4 comma-- 1, 2. So 4 comma 2 is right over here. 4 comma 2. Then we have the point
negative 3 comma 16. So let me draw that over here. So we have negative 1, 2, 3. And we have to go up 16. So this is 2, 3, 4, 5, 6, 7, 8,
9, 10, 11, 12, 13, 14, 15, 16. So it goes right over here. So this is negative 3 comma 16. Negative 3 comma 16. So the line that
goes between them is going to look
something like this. Try my best to draw a
relatively straight line. That line will keep going. So the line will keep going. So that's my best attempt. And now notice, it's
downward sloping. As you increase an x-value,
the line goes down. It's going from the top
left to the bottom right. As x gets bigger,
y gets smaller. That's what a downward-sloping
line looks like. And just to visualize our
change in x's and our change in y's that we dealt with
here, when we started at 4 and we ended at-- or when
we started at 4 comma 2 and ended at
negative 3 comma 16, that was analogous to starting
here and ending over there. And we said our change
in x was negative 7. We had to move back. Our run we had to move in
the left direction by 7. That's why it was a negative 7. And then we had to move
in the y-direction. We had to move in the
y-direction positive 14. So that's why our
rise was positive. So it's 14 over negative
7, or negative 2. When we did it the other way,
we started at this point. We started at this point,
and then ended at this point. Started at negative 3, 16
and ended at that point. So in that situation,
our run was positive 7. And now we have to go
down in the y-direction since we switched the
starting and the endpoint. And now we had to
go down negative 14. Our run is now positive 7 and
our rise is now negative 14. Either way, we got
the same slope.