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### Course: Geometry (all content) > Unit 1

Lesson 2: Measuring segments# Measuring Line Segments

In this example of measuring a line segment, the numbers span across the positive and negative. Remember, line segments and points are the foundations of geometry, so this is an important concept. Created by Sal Khan.

## Want to join the conversation?

- Hello! I can't understand the difference between |-3| and -3. The english is not my language so it is hard to understand the videos .Thanks in advance|(4 votes)
- Those lines mean absolute value so it takes away the negative sign so it's 3(2 votes)

- Q bisects PR, PQ = 3y and PR = 42. Find y and QR(2 votes)
- So if Q bisects PR that makes Q a midpoint of PR

So any distance of the points P or R to Q would be the same as they're in half

PQ + QR = PR

Or since we know that PQ = QR we can reorganize the equation to be

2PQ = PR

substitute PQ and PR

2(3y) = 42

Now to distribute

6y = 42

and to isolate y we can divide both side by 6, keeping the equation equal

y = 7

Now to get QR we just need to substitute PQ(because they're equal)

QR = 3y

plug in y

Qr = 3(7)

Qr = 21

And we can double-check the equality of the equation

PQ + QR = PR

21 + 21 = 42

or

2PQ = PR

2(21) = 42

42 = 42(1 vote)

- can you try and make all the videos not so long?(0 votes)
- If you watch the video entirely then you will know it is worth it.(3 votes)

- what's 10!

(ten factorial)(2 votes) - how come when you subtract -5 - 3= 2? I thought it was -5 - 3= -2 because -5 - 3= -5+ -3=-2! im confused(0 votes)
- I'm not sure what you mean.

a) -5 - (-3) = -5 + 3 = -2

b) -(5 - 3) = -2

c) -5 - 3 = -8(8 votes)

- for the second example how is it 3 wouldn't it be 4(1 vote)
- could somebody please sum up this tutorial on lines for me real quick not enough time to study for my test!(1 vote)
- does khan academy send you emails about your progress(1 vote)
- Yes, they do. You can also check your progress by going to your own profile and then clicking the "progress" tab. There, you can see your activity, focus, what answers you got right, what answers you got wrong, the skills you're working on, etc.(1 vote)

- can u make a video about nth term in algebra plzz?(1 vote)
- and whats an expression?

(Feeling very clueless.)(1 vote)- know what an equation is? It is made of one or more terms that
*can*consist of variables, and has an equal sign (=). Except expressions don't have an equal sign(1 vote)

## Video transcript

- [Instructor] What is BD, so
when they're just saying BD, they're saying literally
the length of segment BD. So they're saying the length
from B, point B to point D. And B is sitting here at
negative two, D here is at five. So you're looking at really the distance between negative two and five. So you literally could just
say, well, that's gonna be five, that's kind of our endpoint,
minus negative two, so five minus negative two is the same thing as
five plus two, or seven. Another way, you could
literally just count it out. Count out the distance, you
go one, two, to get to zero, three, four, five, six, seven,
to get all the way to five. Or you could say, look, I got to get, I got to go two to the right just to get, I got to go two to the
right just to get to zero, and I got to go five more
to the right to get to D, so that's going to be seven. Let's do a couple more, what is BD again? Well, here we're going from one to four. The distance between
one and four is three. And then let's do one more, what is AD? So A is a negative five,
D is all the way at four. So we're going from negative
five to zero, which is five, and then we're gonna go four
more, five plus four is nine. Or you could say, hey look,
we start at negative five, we end up at four, four
minus negative five is the same thing as four
plus five, which is nine.