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## Geometry (all content)

### Course: Geometry (all content) > Unit 15

Lesson 2: Dividing line segments# Dividing line segments: graphical

CCSS.Math:

Watch Sal figure out the coordinates of a point between two other points that give a certain ratio. A graph is given to make it easier to visualize the problem. Created by Sal Khan.

## Want to join the conversation?

- at1:33, i don't understand how you got 1/4 for the ratio 3:1?

i've been doing the lesson this video is for in geometry for awhile now, and the others don't make sense to me either. i started to think it was just getting them to 4:4, since 1 + 3 = 4 and since there was already a 4 in 1/4, you just made it a 1 in 3:1.

but that doesn't always work. for example, problems with the starting ratio of 3:4 for AB had another one of 3:7 or 3/4. i just don't get it.(47 votes)- I'll try to clarify, I am studying Geometry myself but I would like to help :)

First, let's start with an example using ratios. Say that a cookie recipe said that the ratio between flour and sugar is 3:1 cups. That means for every 3 cups of flour, there is 1 cup of sugar. After you add 3 cups of flour and 1 cup of sugar in, you just added in 4 total cups of ingredients! So, if I need to add 3 cups of flour and 1 cup of sugar, and I start by adding 1 single cup of flour, I am now 1/4 finished with putting the ingredients in (because I still need to add 2 more cups of flour and 1 sugar).

Although using this example for this situation may be a bit more confusing, it still works. Like Sal said at0:30, the longer part of the line segment is three times as large (or 3x, as he states it) than the smaller part, x. It's like how we add 3 times as much flour than sugar in the cookie recipe. Now that we know CB is x, and BA is 3x, we can say x + 3x, or x+x+x+x, equals 4x, or 4 units. So, CB is 1/4.

I hope this helped! XD(54 votes)

- How can we get the soution mathematically?(17 votes)
- can i ask what formula will usefor finding ratio?

thanks(14 votes) - is there a way to understand what fraction to use? Why dont we just use the ratio? How do we use the ratio to find the fraction to use in our equation? what if the numbers are different,say 3:4 instead of 3:1, is the fraction then 1/7? I dont understand.(6 votes)
- This one is a little tricky on the first go. The reason they use "1/4" is because a 3:1 ratio is 3 to 1 distance on the line segment given. On a 3:4 ratio, the fraction would would be "3/7", because it would be 3 parts out of 7 total parts on the line segment.

Hope this could clarify!(10 votes)

- How do I tell if it wants me to do C vertical to A horizontal and vice versa because its so confusing(5 votes)
- So each point has an x coordinate and a y coordinate. You want to first look at one and then the other. So if you first look at the x coordinates, you can see that the total length is 16 (the two points are sixteen dashes apart on the x axis). Then you look at the y coordinates for both points, and see the total height is 5 (the points are five dashes apart when looking at the y axis.

We know the ratio is 3:1, so since 3 + 1 equals 4, we want to divide these numbers into four parts. 16 divided into four parts is 4. And 4 divided into four parts is 1. We want the line AB to have 3 of the parts and the line BC to have one of the parts. That will make the line AB to be three times as long as BC.

So if we start at point C, we want to count up 1 and over 4 to get point B. Or we could start at point A and count down 3 and to the left 12. Either way, you get point B in the same place and create a line AB which is three times as long as BC (so point B is 1/4 the distance of the line of AC).(5 votes)

- how does this make sense?(7 votes)
- Where did you get 1/4 from?(4 votes)
- If you have trouble applying ratios and proportions check out the lesson plan here on Khan Academy which teaches you about it by following this link: https://www.khanacademy.org/math/pre-algebra/pre-algebra-ratios-rates(2 votes)

- At2:02, what does Sal mean when the line is at an incline?(3 votes)
- Sal means that this line is steep; meaning that the line in the graph is considered a line with a high steepness, a high slope in other words. 👍🌻

~💙Wonder💦Woman💙(3 votes)

- I do not understand on how to know whether the coordinate is on the upper half of the line segment or the lower half of the triangle. It seems to always switch around on the questions, with no apparent pattern!(3 votes)
- Hi!

Good question, you can do this by comparing the point on the triangle to the midpoint (of the hypotyneus.) The midpoint formula (there is a video explaining it above), is (x1+x2 /2 , y1+y2/2), where the x's and the y's are the coordinates of the points. After finding the midpoint, you can compare it to the desired point you have on the triangle based off whether the midpoint's y axis is greater or less than the selected point.

Hope this helps!(2 votes)

- what is it asks you instead of in a ratio but in times, such as "such that AB, is 5 times as long as BC"(2 votes)
- that is still a ratio, AB/BC = 5/1(3 votes)

## Video transcript

Find the point B on segment AC,
such that the ratio of AB to BC is 3 to 1. And I encourage you
to pause this video and try this on your own. So let's think about
what they're asking. So if that's point
C-- I'm just going to redraw this line segment
just to conceptualize what they're asking for. And that's point A. They're
asking us to find some point B that the distance
between C and B, so that's this distance
right over here. So if this distance is x, then
the distance between B and A is going to be 3 times that. So this will be 3x. That the ratio of
AB to BC is 3 to 1. So that would be the ratio--
let me write this down. It would be AB-- that looks like
an HB-- it would be AB to BC is going to be equal
to 3x to x, which is the same thing as 3 to
1, if we wanted to write it a slightly different way. So how can we think about it? You might be tempted
to say, oh, well, you could use the
distance formula to find the distance,
which by itself isn't completely uncomplicated. And then this will
be 1/4 of the way. Because if you think about
it, this entire distance is going to be 4x. Let me draw that a
little bit neater. This entire distance, if
you have an x plus a 3x, is going to be 4x. So you'd say, well, this is 1
out of the 4 x's along the way. This is going to be 1/4 of
the distance between the two points. Let me write that down. This is 1/4 of the
way between C and B, going from C to A. B is
going to be 1/4 of the way. So maybe you try to
find the distance. And you say, well, what
are all the points that are 1/4 of the way? But it has to be 1/4
of that distance away. But then it has to
be on that line. But that makes it
complicated, because this line is at an incline. It's not just horizontal. It's not just vertical. What we can do, however,
is break this problem down into the vertical
change between A and C, and the horizontal
change between A and C. So for example, the horizontal
change between A and C, A is at 9 right over here,
and C is at negative 7. So this distance right over
here is 9 minus negative 7, which is equal to 9 plus
7, which is equal to 16. And you see that here. 9 plus 7, this total
distance is 16. That's the horizontal distance
change going from A to C, or going from C to A.
And the vertical change, and you could even just count
that, that's going to be 4. C is at 1. A is at 5. Going from 1 to 5, you've
changed vertically 4. So what we can say, going
from C to B in each direction, in the vertical direction
and the horizontal direction, we're going to go
1/4 of the way. So if we go 1/4 in the
vertical direction, we're going to end up
at y is equal to 2. So I'm just going, starting
at C, 1/4 of the way. 1/4 of 4 is 1. So I've just moved up 1. So our y is going
to be equal to 2. And if we go 1/4 in the
horizontal direction, 1/4 of 16 is 4. So we go 1, 2, 3, 4. So we end up right over here. Our x is negative 3. So we end up at that
point right over there. We end up at this point. This is the point
negative 3 comma 2. And if you were really
careful with your drawing, you could have actually
just drawn-- well, actually you don't have
to be that careful, since this is graph paper. You actually could
have just said, hey, we're going to go 1/4 this way. Where does that
intersect the line? Hey, it intersects the
line right over there. Or you could have said, we're
going to go 1/4 this way. Where does that
intersect the line? And that would have let you
figure it out either way. So this point right
over here is B. It is 1/4 of the
way between C and A. Or another way of thinking about
the distance between C and B, which we haven't
even figured out. We could do that using
the distance formula or the Pythagorean theorem,
which it really is. This distance, the distance
CB, is 1/3 the distance BA. The ratio of AB to BC is 3 to 1.