Gestion de données, fonctions

# Fonction linéaire, fonction affine, proportionnalité

## Multiple examples of constructing linear equations in slope-intercept form

Linear Equations in Slope Intercept Form
Back

## Multiple examples of constructing linear equations in slope-intercept form

Discussion and questions for this video
I don't understand why we divide by three at about 7:50. help?
He is basically simplifying the problem 5/6 multiplied by -3/1, -3 and 6 both are divisible by 3 so he simplifies the equation making it 5/2 multiplied by -1 which then results in -5/2
14 Votes
2 Comments
how do you change from point-slope form to slope-intercept?
y - y1 = m(x - x1)
=> y - y1 = mx - mx1
=> y = mx - mx1 + y1
then simplify.

With an example containing actual numbers, you see it's much easier than the steps above imply:
y - 7 = 4(x - 2)
=> y - 7 = 4x - 8
=> y = 4x - 8 + 7
=> y = 4x - 1

Additional resource: http://www.mathwarehouse.com/algebra/linear_equation/point-slope-to-slope-intercept-form.php
20 Votes
1 Comment
Why do we use X as the common variable in algebra? X is also the horizontal axis in graphs.
That's not a coincidence, as you will see when you start graphing algebraic equations.
11 Votes
1 Comment
What is the difference between f(x) and y? Sal said that he assumed they were equal, but can there be cases where they aren't? How would that work?
They're the same thing, but in higher math classes you use f(x) because you graph functions and y can be used as a variable. Say you have a question that says
f(x)=3x + 4, what is f(2), you would simply plug in 2 for x and get 10. It's much more useful in more advanced math.
You can also watch this video.
https://www.khanacademy.org/math/cc-eighth-grade-math/cc-8th-relationships-functions/cc-8th-function-notation/v/difference-between-equations-and-functions
At first when you are learning functions, you would think that they are silly and you can use y instead. But later on, you will realize that y is silly.
I hope this helps!
12 Votes
1 Comment
Why is 6/-3 equal to -6/3 or -2?
6/-3 means 6 ÷ -3
A positive divided by a negative is a negative, and 6 ÷ 3 = 2.
Therefore 6/-3 = -2

-6/3 means -6 ÷ 3
A negative divided by a positive is a negative, and 6 ÷ 3 = 2
Therefore -6/3 = -2

-2 = -2

Since 6/-3 = -2, -6/3=-2, and -2=-2 (i.e. they are all equal to the same thing), they must also be equal to each other.

This same process could be done for any numbers meaning that
(-x)/y = x/(-y) = -(x/y)
or in words: A negative in a fraction can go in front, in the numerator, or in the denominator, but not both.
4 Votes
1 Comment
at 3:27 shoudnt it be y2-y1 over x2-x1 instead of y1-y2 over x1-x2
Believe it or not, they are the same thing. Try it.
8 Votes
2 Comments
to find the slope sal subtracts (at 3:00) 6-0/2-5. check. next problem i paused, tried to figure it out myself and subtracted (at 5:54) 5-0/3-(-3) to find the slope. sal however reversed the order and did 0-5/-3-3. we ended up with very different answers. how do i know which point to subtract from which point? he did them in two different orders.
Ramona, it looks like you are on the right track. Continuing from your set up, you should get this result:
slope = (5-0)/(3-(-3)) = 5/(3+3) = 5/6

That's the same answer Sal got to in the video.

To answer your question about knowing which point to subtract from which point - either way is correct! It is _your choice_ which is the 'first' point and which is the 'second' - just make sure you make the _same_ choice for both the x and y coordinates. In the video, Sal chose to set up the calculation 'green minus orange' and you set it up 'orange minus green'. But either way, once all the arithmetic is done, you should have the same answer. (Why? Note what happens at 7:01: _"...the negatives cancel out."_
At 8:37, if b in the equation y = mx+b is 5/2 or 5 halves, how would it be plotted on the coordinate plane in fraction form as the y intercept? Isn't the y intercept supposed to be a whole number, so that it can be plotted on the coordinate plane?
The y-intercept can be any number, it need not be a whole number (usually is not). You just plot 2.5 as best you can, half-way between 2 and 3.
4 Votes
3 Comments
This isn't exactly related to the topic, but why does 6-0/2-5 become -6/-3 ?
Why isn't it just 6/-3 ? I never understood this and it always stuffs me up.
Tomm0,
Your mistake is that (6-0) is +6, not -6
If you had a line that included the points (6,2) and (0,5), the slope would be (change in y)/(change in x) which would be (6-0)/2-5)
(6-0) is +6 , not -6
(2-5) is -3
So the fraction is 6/-3
which reduces to -2/1 = -2

I hope that helps make it click for you.
4 Votes
1 Comment
How Would You Work This Type Of Problem When It's Set Up In A Table???
3 Votes
2 Comments
do you mean if its like
x-2/3/4/5/6
y-4/6/8/10/12 you have to find a pattern (in this case y is twice the amount of x) so y=2x
I don't get it. At 3:39 is confusing. Why can't we use the slope equation to get the slope and then the slope would be 2 not -2. Am I missing something? He said if it's switch then it would be be negative?
I think I understand what you are trying to say, if you are talking about the equation y2-y1/x2-x1. So you would have 0-6/5-2. First, if we think about it, the slope will be the same no matter if it is from (2,6) to (5,0) or (5,0) to (2,6). Just know the slope is the same, so we can do it Sal's way of 6-0/2-5 or the way 0-6/5-2. You probably thought that 0-6 was just 6, but 0-6 is -6. Then you get 5-2 which is 3, so -6/3. If you thought 0-6 was 6, you would have got 2 as the slope. I think that is what you are asking
Comment if you get it or if you meant something else
write th equation of the line in slope- intercept form if the slope is 1/5 and the y-intercept is -9
when you put an equation in y intercept form you always start with y= the slope goes with the x and the intercept is the next term.
y = mx +b
m is the slope, b is the intercept
y= 1/5x -9
What if the slope is undefined, but you are given a point. What do you do?
Hannah,
If you have a linear equation where the slope is undefined, then the denominator of the slope must be 0 since anything divided by 0 is undefined.
The denominator of the slope is the change in x
So the change in x is zero.

So if your point was (1,3) and the slope is undefined, you know that some other point has a change in y but x changes by zero,
So x would always by 1.
Points (1,3), (1,4), (1,5) would all be on your line.
The equation of this line is x=1+0y or x=1
If you graphed it you would have a vertical line. going through the x axis at the point given as the x value in your original point.

I hope that makes it click for you.
what do i do when the equation is Y = X
it means that whatever your X value is,that will also be your Y value. when X=1 Y=1 when X=5 Y=5.and therefore the slope is 1(1/1). it follows the usual equation of a line Y=mX + b.when X is 0 so is Y, that's why the Y intercept (b) isn't in the equation, its 0.
AT 2:23 I don't get how he got -x
Try watching the video again.
How do I write the slope-intercept form of the equation of the line through: (3,-2), if the slope is undefined?
Hi,
If a slope is undefined, it means that the x value does not change. Remember that slope is calculated as the change in the y value divided by the change in the x value. If x does not change, then we are dividing by zero, which is undefined. Therefore, an undefined slope is a vertical line through the point you were given.
You cannot really write it in a correct slope/intercept form. Usually this is written as:
x=whatever the x value of your point is.
In your case, the equation would be
x=3
And the graph would be a vertical line running through the 3 on the x axis.
Hope that helps :-)
I didn't understand the equation f(1.5)=-3,f(-1)=2?
This is because our function here is
*f(x)=(x)-2*
So each time we put in a value of x we multiply it by -2
Ex.
f(1.5)=(1.5)*-2=-3
f(-1)=(-1)*-2=2
how do you find a y intercept in a graph
i don't understand what is going on when you have to solve for b, can i please have some help?
When solving for b we put the values (x,y) of the given point (which sit on the line) in the the equation of the line in its general form: y=mx+b, where m is our slope (in this example -1) Than we separate b on one side of the equation and numbers on another.
how is it " m=delta y over delta x? " why is it not delta x over delta y?
slope can also be though of in terms of rise over run. since delta y is how far the graph is rising and delta x is how far the graph "runs", thats why the slope is delta y over delta x. It is just a mathematical convention that slope is measured that way.
Is it m=y2-y1 over x2-x1 or m=y1-y2 over x1-x2? I need help. Please and Thank You.
It really doesn't matter. As long as you have the y's over the x's, you'll get the slope of your line. In fact, the two ratios are equal anyways because y1-y2=-(y2-y1), annd the same for the x's, so multiply one way by -1/-1, and you'll get the other.
About the substitution of the points. What if you don't have a zero? Do you just go with the points that are closest to one?
@ 1:07} Do you have to use m to represent the slope or do you choose any variable?
Well technically, you have to use m to represent the slope or else it wouldn't be right.
How do you make it easier to convert from a problem such as -5x + 2y = 6 without getting confused over negative numbers?
Tommy,

Typically to isolate y (so you can get into the form y = mx + b) you would start this problem by adding 5x to both sides (to get rid of the -5x on the left). This first step would give you:

2y = 5x + 6

Bingo. Your negative sign is gone. Now a you divide both sides by 2:

y = 5/2x + 3 ... and you're done!
at 3:53 , why does Sal put a negative when 6 - 0 is not negative?
because he went the other way around just to prove you would get the same value for the slope.
how do i find an equation of the slope line?
I do not get your question.
it is y1 -y2 over x1- x2
How do you solve a problem when it asks you, " Write the equation that describes each line in slope-intercept form." and the problem is [slope= -2/7, (14,-3) is on the line]? PLEASE ANSWER SOON OR PUT A VIDEO UP ON KHAN ACADEMY ABOUT MY QUESTION!!!!!!!
well look at it this way: slope= -2/7, (14,-3), look at in the video 10: 11 through the rest of the video, I hoped this helped you Sophia.
And when graphing these can if you can switch the 6/-3 around will it change the graph points
Stephanie,
If I understand your question correctly, you are asking how to find slope. If you are given points, (example: 1,2, 3,4) then you can find your slope using the equation:
Y2 - Y1 over X2-X1.
In this case, Y2 is 4 because this is the second y in the two points, Y1 is 2, X2 is 3, and X1 is 1. We now have: 4-2 over 3-1 which is 2 over 2 which can be simplified to 1. The slope of the equation is 1.
Hope this helped!
Isn't -2-1=3 ? Sal said that it equals to 2?!
The answer is not 2. Probably just a simple mistake
2 Votes
1 Comment
if you have 2lbs of jellybeans for $9, what of .5 lbs? 3lbs? 4 lbs? I would solve this using proportions: if you have the proportion$9 per 2 lbs., you can divide both by 2 to get $4.50 for 1 lb. From here you can multiply both by 3, 4, or 1/2 to get your answers. I'll leave that up to you ;D Ok, I'm bringing this up because I have noticed this over the past year. I mean no offense, I merely want to have help! The videos that are supposed to provide "help" are often confusing to me, and ultimately cover things I already know, and/or just confuse me further. They, in the long run, don't actually clarify the work I am supposed to do, and this frustrates me. Is there anything that can be done about this? Please help!! Thank you!! - Grace Well, you can go on to Algebra 2. Sorry if this didn't help. what is the *slope-intercept formula* and x and y are points on the graph. at 1:48 how did he find the y-intercept of the equation if it was not written in standard form ? 2 Votes 1 Comment He knew that line goes through the point (4/5, 0), right? 4/5 is an x value and 0 is an y value of this point. Also he knew that slope (m) is equal to -1. Therefore by substituting the values to the formula of y = mx + b we get 0 = -1 * 4/5 + b which is an equation with one variable and just like Sal did - 0 = -4/5 + b therefore b = 4/5. i really do not understand this video. how would you write in a equation form: Jan wants to buy maps and atlases. the maps cost$2 each and the atlases cost each $5. If she buys 3 atlases and spends$25, how many maps can she buy?
Can you plze explain how to write an equation with this example?
An equation for this is
t = 2m + 5a
where t = total, m = maps, a = atlases

You're also told that a = 3 and t = 25, so
25 = 2m + 5 * 3
Solve for m.
Will you please show how to find the slope-intercept form for the line satisfying the following:
x-intercept 3, y-intercept 2/3?
Well remember x represent the horizontal line and so your first point will be (3,0) while the vertical line (y) represent a point in y lower than 1 (dividing 1 in thirth so you take 2 of them and you have 2/3). Finally your second point is (0,2/3) and join the dots.
m= 3 - 0/0 - 2/3= -9/2 = -4 1/2... y= -9/2x+2/3 (mx+b)
Sorry if I was a little obvious but my intention was be clear!
How do you write the the equation if its a word problem?
Write down what you know and what you don't know and what it is asking for clearly. Try to find a relationship. Remember that all lines have a slope (m), which is basically how steep it is, which is simply a measure of how far it moves along the x axis over how far it moves along the y axis, and a point of origin where it crosses a known point. In normal notation, this is the y intercept (b), which occurs when x = 0.
what is the x for ?
Its the value of the number on the x axis.

Oh yeah, - x is just a variable.
If we can plot an equation in which no x is defined ( e.g. 2y=-8 where we assume 0x) why can we not graph an equation in which no y is defined (e.g. 4x=-8)
You can graph 4x=-8. When you divide both sides by 4 you get x=-2. This is simply a vertical line which crosses the x intercept at -2. That vertical line includes all points in which x is equal to -2.
Is there a particular reason why Sal used less common variables in this video? Or did he just want to through in a variety?
Well, if you are talking about him using the variables: m, x, y, and b, he is using the equation of slope-intercept form, which is *y = xm + b*. "b" is the y-intercept and "m" is the slope of the line, and x + y are points on the line, of course. This equation makes a straight line. If you want to know more, just look here: https://www.khanacademy.org/math/algebra/linear-equations-and-inequalitie/equation-of-a-line/v/graphing-a-line-in-slope-intercept-form.

Also, the f at around 10:32 is a symbol for function.
at 2:57 in the videos sal used these triangular shaped things in the equation what are they and what do they represent
They are the Greek symbol delta. They are accepted in math and science as the word "change".
3 Votes
1 Comment
How do you write an equation in slope intercept form?
A linear equation in slope-intercept form is y = mx + b. Where "m" is your slope and "b" is your y-intercept.
How to graph y=x
The y-intercept would be zero and the slope would be 1.It would have a constant increase and go in a straight equal line.
Hi you have done that in a problem
Line contain points (2,6) and (5,0)
m=y1-y2/x1-x2

but the formula is
http://cs.selu.edu/~rbyrd/math/slope/slo_eq1.gif
Since (x1,y1) and (x2,y2) could be either of the two points, it it true that is doesn't matter which way the slope formula is written. It is important that you use your original point for both the numerator and denominator or your slope will have the opposite sign it should have.
Around 4:12, Sal had a problem which said,

"The line contains points (2,6) and (5,0)"

In this problem, Sal subtracts this way, "6-0/2-5"

In the next problem 6:48 Sal does his problem "The line contains points (3,5) and (-3,0)" and SWITCHES AROUND compared to his problem before, by subtracting "0-5/3 - -3".

I'm not a mathematical genius compared to Sal so please help me see where I'm going wrong. I thought the problem would turn out like this,

"5-0/ -3-3"

Sal's Equation: "0-5/ 3- -3"
Christian, that's ok! You see, when you want to do these problems, you usually have two sets of points, right? Let's call them (X1,Y1) and (X2, Y2). Now, in school, usually the general formula taught for finding slope is (Y2 - Y1) divided by (X2 - X1). But, as you'll learn later on (if you aren't taking higher level math yet) that the equation is actually "CHANGE OVER Y" divided by "CHANGE OVER X". The order in which you subtract doesn't matter, as long as you subtract the Ys and divide them by the subtraction of the Xs. If you'll notice, your equation gives you the answer "5/-6," which is "-5/6". Sal's answer is also "-5/6".
Hope that helps!
what does delta mean?
Delta is a Greek letter: http://en.wikipedia.org/wiki/Delta_(letter).

Mathematicians use the letter delta to represent a change in value of a variable. For example, ∆x is the change in the x variable. So when you see ∆y/∆x you know you're dealing with slope as it's the change in y compared to (or over) the change in x. This is also called rise over run: rise is how far you change vertically. Run is how far you change horizontally.
what is linear equation
Yogi,
A linear equation is an equation which graphs to form a line.
It usually takes the form of y = ax+b where a and b are constants such as y=2x+1.
When you graph it, it forms a strait line, so they call it a linear equation.
Here is a video that might help you understand more:
http://www.khanacademy.org/math/algebra/linear-equations-and-inequalitie/graphing_solutions2/v/algebra--graphing-lines-1
How are you supposed to find the fraction
aka the answer?! I need help.. any1?
IM THE SHORT GUY PEOPLE PLZ HELP!.... and thx and i wonder who else needs help ..
How do i turn x+2y=5 into slope intercept form? PLEASE HELP
TCulton,
The slope intercept form is y=mx+b
You have the form x+2y=5
In the slope intercept form, the y is all by itself on the left. So convert
x+2y=5 to a from that has y by itself on the left side of the equation
x+2y=5 First get rid of the x on the left by subtracting x from both sides.
x-x+2y=5-x so
2y=5-x Now get rid of the 2 by dividing each side by 2
2y/2 = 5/2 - x/2 so
y=5/2 - x/2 Now change the order on the left
y=(-1/2)x+5/2
So it is now in slope y-intercept form. The slope is -1/2 and the y-intercept is the point (0,5/2)
2 Votes
4 Comments
At 7:20 why did he choose the second point, what if the first piont??
It doesn't matter which point you select. Sal probably picked the 2nd point because the zero usually makes some of the math easier to do. If you were to use the 1st point to find "b", you would get the same value for "b". Here it is:
5 = 5/6(3) + b
5 = 15/6 + b
5 = 5/2 + b
10/2 - 5/2 = b
5/2 = b
how do i make 6x - 3y = -9 in slope intercept form
Isolate y:

6x - 3y = -9, add 3y to both sides
6x = -9 + 3y, then add 9 to both sides
6x + 9 = 3y, then divide everything by 3
2x + 3 = y

you can always enter this kind of stuff into wolfram alpha and they will solve it and graph it for you: http://www.wolframalpha.com/
how do i solve 6x - 3y = -9 for y
check my previous answer, but be aware that the value of y depends on x. Every solution is a point that is on the line 6x - 3y = -9
i dont understand why negatives cancel out
A negative is the opposite of a number. The opposite of 2 is -2. The opposite of -2 is 2. Two negatives means 'the opposite of the neg. number', which is positive. Therefore --2=2. Three negatives (---2) is the opposite of the opposite of the opposite of 2.
Why does he subtract the two points when one is greater in x and y values than the other?
Humza,

For some reason, I can't access the video right now, but what Sal was probably trying to do was to calculate the slope of the line from two points that are on the line. In order to do that, the formula is m=(y2-y1)/(x2-x1). Basically that means that you call one of the points, "point 1" and the other "point 2". Then you subtract point 1's y value from point 2's, and divide that by point 2's x value minus point 1's x value.
2 Votes
3 Comments
i cant figure out what (34,87) (51,25) can u plaese help me
I know that the slope is the change in y divide by the change in x. But what are you getting, if anything, if you divided the change in x by the change in y?
George,
If you take the *negative* of change in x over change in y, you would have the slope of a line perpendicular to the original line.

This video explains why: https://www.khanacademy.org/math/algebra/linear-equations-and-inequalitie/more-analytic-geometry/v/perpendicular-lines
2 Votes
3 Comments
I thought it was y2 - y1 over x2 - x1.
and it is, the short way to write it like you did is just to put the little triangle before the x and the y. tha just means change!
At 3:28 why does he put the first coordinate first? the equation is y>2- y>1
x>2- x>1
You can pick either one to be honest. you can start with the first coordinates or the second coordinates it doesn't matter the order.
how would you determinr the the rate of change is constant for tables given.
Spencer,
If the
(change in y) / (change in x) is always the same, then the rate of change is constant.

If you table has the following
(3,5)
(4,7)
(6,11)
(7,14)
You could calculate (change in y)/(change in x)
For the first two terms (7-5)/4-3) = 2/1 = 2
The second & third term (11-7)/(6-4) = 4/2 = 2
So far the change is constant
The third and fourth term (14-11)/(7-6) = 3/1 = 3
This last point in our table shows we have a different ratio for
(change in y)/(change in x) so the table does not have a constant rate of change.

I hope that helps make it click for you.
what does f(x) - f of x mean?
f=function. If you have an equation like y = 3x + 7 then you can also say that y is a function of x, or f(x). f(x) = 3x +7.
Sal, you kept saying that the points listed MUST satisfy the equation, what if the points do not satisfy the equation, I'm just wondering?
Points that dont' satisfy the equation cannot be on the line.
Sorry, could someone explain to me why you multiply x by the slope?
When x increases by 1, y increases by the slope m.
Consider the points (0,b) and (x,y).
Here the first coordinate increases by x and therefore the second coordinate should increase by m times x:
y = m*x + b
When finding the slope from the points could you just stack and subtract? Would that work everytime for points?
that was the way i was taught. it never gave me any trouble before. that math is much easier to do in your head anyway and maybe only need to write one or two numbers down till you get the answer.
what if all you have are two points? how would you find the y-intercept? I still don't understand!!
Two points define a line, and if you define a line, then you define the y-intercept. If you have two points, you can calculate the slope of a line between them. Then you can write y = mx + b, and you know x. Plug in the y and x of one point, and solve for b. That's the y -interecept.
Please explain how to graph the equation and state the slope of the line if the slope existed Example x=-8
x = -8 is a vertical line, because it says no matter what the y value is, x will always be -8.

The slope can be said to be "infinite", but since infinity is not a number, the slope in this case is "undefined", and that's what you should put down when taking a test.

The graph, once again, is just a vertical line extending in both directions forever, at x = -8.

Hope I helped!
I don't get this?
Help!
Well, It'd help if you told us *what* you don't get..
Just a suggestion.
- tempest
2 Votes
3 Comments
For the Slope, my teacher told me to take Y sub two minus y sub one, and the same for x. however, it looks like Sal did y sub one minus y sub two for one of the problems. can you do that or does it matter which way you do it?
Either way is fine. The only thing to be sure of is that you're consistent. If you did y₂-y₁ and x₁-x₂ you'd get the opposite of the slope, so make sure that one point either comes first or second both times.
How would you graph negative 5x + 6 if you wanted to?
Are you saying the variable is negative like "-5x+6" or the whole expression is negative like "-(5x+6)"? Because these are two completely different.

-5x+6 has a slop of -5 (meaning that the line goes up 5 units in the y-direction while going left 1 unit in the x-direction) and the first point you would graph, starts at (0,6) which is 6 on the y-axis.

-(5x+6) however means the whole expression is being multiplied by -1. Which then means you would get -5x -6. This would mean you would have the same slope as the previous (up 5 units in the y-direction and 1 unit to the left in the x-direction) but it also means the first point to graph would be -6 instead of +6.

This would result in two lines which have the exact same slope (and therefore parallel) but are 12 units apart in the y-direction (meaning one starts out at +6 on the y-axis and the other starts out at -6 on the y-axis.

Hope this helps!
At 6:39, i understand that Sal started with the second set of coordinates (-3,0) first but, i thought it didn't matter. I later tried doing the equation with the first set of coordinates first (3,5) and it doesn't work because you end up dividing 5 by 0. This doesn't work. Can someone explain how to know which coordinate set to use first? I am a little confused. Thank you, Elena
You were initially right: the order doesn't matter when finding slope.
(5 -0)/(3--3) = 5/6
(0-5)/(-3-3) = -5/-6 = 5/6
Both give the same answer.
2 Votes
1 Comment
At 6:11, how come its 0-5 instead of 5-0? Is that crucial to get the correct answer or does it not matter, because you are still finding the change?
(y2 - y1)/(x2-x1) works either way. Just make sure that whatever point you started with for the difference in y in the numerator is the same point you start with for the difference of x in the denominator.
2 Votes
1 Comment
Does anyone know WHY this works?

And why is it always y=x+(number) or y=x-(number). Why not just have say, 3 coordinates? So say x,y,s. (s for slope).

And why does y=y/x+(number). I get it in terms of algebra but why do we solve it the way we do?
If you have a 2-dimensional coordinate plane, the y = ax + b is the form of the equation that will graph as a straight line. It's not a choice, that's just the way it is. Pick a and b. Let's say you pick 3 and 8. Now you have y = 3x + 8. That's an equation of a straight line. Try it and see. Put in a bunch of numbers for x. Calculate y for each one. Graph each of those points. You will see a straight line