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Current time:0:00Total duration:5:24

CCSS Math: HSF.IF.B.4

- [Instructor] We're told
Kaya rode her bicycle toward a tree at a constant speed. The table below shows the relationship between her distance to the tree and how many times her front tire rotated. So once her tire rotated four times, she was 22 1/2 meters from the tree. Then once she's rotated eight times, she's 12 1/2 meters from the tree. When it rotated 12 times, then she's only 2 1/2 meters to the tree. So she's getting closer and closer the more rotations that her tire has had. So then they ask us some questions here. So they ask us how far away
was the tree to begin with? How far does Kaya travel
with each rotation? How many rotations did it
take to get to the tree? So like always, pause this video, and see if you can answer these questions on your own before we do it together. So let's start with the first question. How far away was the tree to begin with? So the way that I would think about it is, after four rotations, we're
22 1/2 meters from the tree. We see that if we increase
by another four rotations, so let's say plus four rotations, we see that we have gotten
10 meters closer to the tree, or our distance to the tree
has gone down by 10 meters. So I'll write negative 10 meters here. If we want to figure out how
far the tree was to begin with, we have to go back to zero rotations. So if we're going back by four, so we're subtracting
four from the rotations. And if we're going at a constant rate, well, then we would add 10 meters. So we would add 10 meters. If we add four rotations,
we get 10 meters closer. If we take away four rotations, to get us back to zero rotations, then we will go 10 meters further. So that would be at 32.5 meters. So 32.5 meters is how far the tree was to begin with when Kaya
had zero rotations. The next question is
how far does Kaya travel with each rotation? All right, well, we already
saw that with four rotations, she's traveling 10 meters, so we could say 10 meters in four rotations. So if we divide both of these
by four, what would we get? Well, that's the same thing as
2 1/2 meters in one rotation. So this is 2.5 meters. And last but not least, how many rotations did it
take to get to the tree? Well, we know that after 12 rotations, she's only 2 1/2 meters
away from the tree. We also know that in every one rotation, she gets 2 1/2 meters closer. So she only needs one more rotation to cover this next 2 1/2 meters. So, if we go plus one rotations, we're going to go down 2 1/2 meters. We're going to go 2 1/2
meters closer to the tree, and we will be at the tree. So how many rotations did it take to get to the tree in total? 13 rotations. Now one thing that's
interesting is to think about what we just did in a graphical context that you might have seen before. And if we were to put on the
horizontal axis rotations and if we were to put on the vertical axis distance to the tree, distance to tree, I'll just call the
vertical axis the y-axis and the horizontal axis the x-axis, well, we could see here
that we have zero, four, eight, 12, I could go to 16. And then we saw that at zero rotations, we are 32.5 meters from the tree, so 32.5. This is all going to be in meters. So this first question was
really another way of asking what is our y-intercept? And this next question, how far does Kaya travel
with each rotation, well, we saw that when you
increase your rotations by four, your distance to the tree
goes down by 10 meters. So when this is plus four, we went down, we went down 10 meters. So it's negative 10 meters. So really what we were thinking
about right here, this is, you could think about the
magnitude of the slope. The slope of this line, the slope of this line that
would describe her distance to the tree as based on
the number of rotations, the slope is going to be
our change in our distance, which is negative 10, four, our change in rotations, over four. So the slope of this line is negative 2.5 meters per rotation. But when they say how far does Kaya travel with each rotation, she's
getting 2 1/2 meters closer. Her distance from the tree
goes down by 2 1/2 meters. And this last question, how many rotations did it
take to get to the tree, well, at what point is our y value, our distance to the tree, zero? And we saw that it is at 13 rotations. So this is another way of thinking about what was the x-intercept? So the line, it's a line because we
know that she's traveling at a constant rate, looks
something like that. So they really were
asking us the y-intercept, the slope, and the x-intercept.