## Video transcript

What I want to introduce you to
in this video is the notion of a proportional relationship. And a proportional relationship
between two variables is just a relationship
where the ratio between the two variables is always going to be the same thing. So let's look at an example of that. So let's just say that we want
to think about the relationship between x and y. And let's say that when x is one, y is three, and then
when x is two, y is six. And when x is nine, y is 27. Now this is a proportional relationship.
Why is that? Because the ratio between y and x
is always the same thing. And actually the ratio between y and x
or, you could say the ratio between x and y, is always the same thing. So, for example--
if we say the ratio y over x-- this is always equal to--
it could be three over one, which is just three.
It could be six over two, which is also just three. It could be 27 over nine,
which is also just three. So you see that y over x is
always going to be equal to three, or at least in this table right over here. And so, or at least based on
the data points we have just seen. So based on this, it looks like that
we have a proportional relationship between y and x. So this one
right over here is proportional. So given that, what's an example of
relationships that are not proportional. Well those are fairly easy to construct. So let's say we had-- I'll do it with
two different variables. So let's say we have a and b. And let's say when a is one, b is three. And when a is two, b is six. And when a is 10, b is 35. So here-- you might say look, look when a is one, b is three so the ratio b to a-- you could say b to a-- you could say well when
b is three, a is one. Or when a is one, b is three. So three to one. And that's also the case when b is six, a is two.
Or when a is two, b is six. So it's six to two. So these ratios
seem to be the same. They're both three.
But then all of sudden the ratio is different right over here. This is not equal to 35 over 10. So this is not a
proportional relationship. In order to be proportional
the ratio between the two variables always has to be the same.
So this right over here-- This is not proportional. Not proportional. So the key in identifying
a proportional relationship is look at the different values
that the variables take on when one variable is one value, and then what is the
other variable become? And then take the ratio between them. Here we took the ratio y to x,
and you see y to x, or y divided by x-- the ratio of y to x
is always going to be the same here so this is proportional. And you could
actually gone the other way. You could have said, well
what's the ratio of x to y? Well over here it would be one to three, which is the same thing as two to six, which is the same thing as nine to 27. When you take this ratio--
if you say the ratio of x to y instead of y to x, you see that
it is always one third. But any way you look at it--
the ratio between these two variables-- if you say y to x,
it's always going to be three. Or x to y is always going to be one third. So this is proportional
while this one is not.