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### Course: 8th grade > Unit 3

Lesson 13: Linear and nonlinear functions- Recognizing linear functions
- Linear & nonlinear functions: table
- Linear & nonlinear functions: word problem
- Linear & nonlinear functions: missing value
- Linear & nonlinear functions
- Interpreting a graph example
- Interpreting graphs of functions
- Linear equations and functions: FAQ

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# Recognizing linear functions

Learn to recognize if a function is linear. Created by Sal Khan and Monterey Institute for Technology and Education.

## Want to join the conversation?

- So the non-linear function in this video is a parabola?(11 votes)
- In Sal's table, notice that every value of y equals 10 plus x^2

When x = 1, y = (1)² + 10 = 11

When x = 2, y = (2)² + 10 = 14

When x = 3, y = (3)² + 10 = 19

When x = 4, y = (4)² + 10 = 26

When x = 5, y = (5)² + 10 = 35

So, in each case shown in the table, y = x² + 10 and that is definitely a quadratic. A quadratic describes the points that make a parabola.

Technically, though, we don't know if this function is**continuous**or if it is defined by that table and**only**has`those 5 points`

. Sal only said that the function contains those points and no one tells us that there are any other points in the function. We haven't been told if x = 0 is included or x = 1/2 or x = -3

Anyway, those points in the table**do**lie on a parabola--we just don't know if there are any points between those.**If**the problem said that the function was defined by

y = x² + 10, or if it showed the curve of a parabola with those points on it, then we would know that all the points were included. . . but then the video wouldn't be making Sal's point which is how you can know that a function is**linear**just by looking at the table and this one is definitely not linear.(45 votes)

- So would a function with the following points be a linear function?

(1,1)

(2,4)

(4,7)

(8,10)

(16,13)

(32,16)

The change in x is constant, it's always x times 2.

The change in y is constant, it's always y plus 3.

But when these points are plotted on a graph, there is no straight line between them?

How can you have a constant change in x and y but a nonlinear function?(10 votes)- well, you are not having a constant change in x and y.

To go from x = 1 to x = 2, you add 1. to go from y = 1 to y = 4, you add 3. it's okay for now. But to go from x = 2 to x = 4, you add 2, so you should add 3*2 =6 to the previous y (i.e.,4) to get 10, but you added only 3 to get 7.(2 votes)

- how do I know if a function is linear or not when it is explained like this: f(x)=x-11; (4) ?(8 votes)
- When the variable (x) has a exponent of 1 (or you do not see one because 1 is understood) it represents a line which is length. Length is usually in units of cm, m, etc...(1 vote)

- Would something like y=3 be linear or nonlinear?(5 votes)
- Any equation in the format y=n(n stands for a number) or x=n will be linear.(3 votes)

- So can negative number also be linear or is that just for positive numbers(5 votes)
- Linear equations can have negative values in them! For example:

x y

-2 -5

-1 -3

0 -1

1 1

This set of values is linear, because every time x increases by 1, y goes up 2 so there is the same interval between each y value. This works even though there are negative numbers!(5 votes)

- I still don't get what a linear functions is?(3 votes)
- A linear function refers to when the dependent variable (usually expressed by 'y') changes by a constant amount as the independent variable (usually 'x') also changes by a constant amount. For example, the number of times the second hand on a clock ticks over time, is a linear function. Every minute (the constant change in x) the second hand ticks 60 times (the constant change in y). This is a linear function because for every 1 minute, the clock ticks the same number of times. If we express this situation on a graph, we would observe a straight diagonal ray, starting at (0,0) and increasing towards the upper right. As x (minutes) increases by 1, y (number of ticks) would increase by 60.(6 votes)

- I would ask for help but are people still on here to ask?(3 votes)
- Yes, I would love to help I am not good at math but I can try and help you a little! What do you need help with? I will give you the most help I can!(5 votes)

- i dont understand like the x and y dont dont agree with there constants(5 votes)
- At0:46you talk about seeing if it's Linear by dividing the change in Y by the in change X. I did not understand that?(2 votes)
- It means he is dividing the amount between the Y numbers by the amount between the X numbers.

Example:

X Y 14-11= 3 2-1=1

1 11

2 14 3/1= 3(5 votes)

- can negatives by linear(3 votes)
- Yes. It doesn't matter if a line is negative or positive as long as the change in y over the change in x is constant.(1 vote)

## Video transcript

Deirdre is working with a
function that contains the following points. These are the x values,
these are y values. They ask us, is this function
linear or non-linear? So linear functions, the way to
tell them is for any given change in x, is the change
in y always going to be the same value. For example, for any one-step
change in x, is the change in y always going to be 3? Is it always going to be 5? If it's always going to be the
same value, you're dealing with a linear function. If for each change in x--so over
here x is always changing by 1, so since x is always
changing by 1, the change in y's have to always
be the same. If they're not, then
we're dealing with a non-linear function. We can actually show
that plotting out. If the changes in x-- we're
going by different values, if this went from 1 to 2 and then
2 to 4-- what you'd want to do, then, is divide the change
in y by the change in x, and that should always
be a constant. In fact, let me write
that down. If something is linear, then
the change in y over the change in x always constant. Now, in this example,
the change in x's are always 1, right? We go from 1 to 2, 2 to
3, 3 to 4, 4 to 5. So in this example, the change
in x is always going to be 1. So in order for this function to
be linear, our change in y needs to be constant because
we're just going to take that and divide it by 1. So let's see if our change
in y is constant. When we go from 11 to
14, we go up by 3. When we go from 14 to 19, we go
up by 5, so I already see that it is not constant. We didn't go up by 3 this
time, we went up by 5. And here, we go up by 7. And here, we're going up by 9. So we're actually going up by
increasing amounts, so we're definitely dealing with
a non-linear function. And we can see that if
we graph it out. So let me draw-- I'll do
a rough graph here. So let me make that my vertical
axis, my y-axis. And we go all the
way up to 35. So I'll just do 10, 20, 30. Actually, I can it do a little
bit more granularly than that. I could do 5, 10, 15, 20,
25, 30, and then 35. And then our values
go 1 through 5. I'll do it on this
axis right here. They're not obviously the exact
same scale, so I'll do 1, 2, 3, 4, and 5. So let's plot these points. So the first point is 1, 11,
when x is 1, y is 11. This is our x-axis. When x is 1, y is 11, that's
right about there. When x is 2, y is 14, that's
right about there. When x is 3, y is 19,
right about there. When x is 4, y is 26,
right about there. And then finally, when x is 5,
y is 35, right up there. So you can immediately
see that this is not tracing out a line. If this was a linear function,
then all the points would be on a line that looks something
like that. That's why it's called
a linear function. In this case, it's not,
it's non-linear. The rate of increase as
x changes is going up.