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## Algebra basics

### Course: Algebra basics > Unit 8

Lesson 1: Equations & geometry- Equation practice with segment addition
- Equation practice with segment addition
- Equation practice with midpoints
- Equation practice with midpoints
- Equation practice with vertical angles
- Equation practice with vertical angles
- Equation practice with complementary angles
- Equation practice with supplementary angles
- Equation practice with angle addition

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# Equation practice with midpoints

Given an information about the distance of a segment's midpoint from its endpoints, Sal forms and solves an equation in order to find the length of the segment. Created by Sal Khan.

## Want to join the conversation?

- how do you find the end point B if they give you the coordinates to A and the midpoint M

eg. A(7,-3) M(1,-1) find the endpoint B(9 votes)- First multiply the midpoint by 2 getting you (2,-2) then subtract the midpoint from A for x 7-2 and y -3--2, the difference (5, -1) is B(5 votes)

- Around2:00, 8x - 7x = x? I assume the missing 1 is just a place holder?(4 votes)
- why are the subtitles Japanese.(3 votes)
- Did you try changing the language of the subtitles?(4 votes)

- Wait, 8x - 7x would be 1 so wouldn't x be 3 instead of 2 ?(4 votes)
- X X X X X X X X - X X X X X X X = X

That's why X is equal to 2 in this example

We subtracting seven X'es from eight X'es and getting only one X left. 7 and 8 is just a multiplier.(1 vote)

- in this video, Sal only explains what to do when you have to subtract. What do you do when you need to add such as:
`4x + 7 = 3x + 14`

?(3 votes)- Here you can subtract 7 from both sides to get 4x=3x+7. Then you subtract 3x from both sides of the equations to get x=7.(3 votes)

- So, what if the question gave you the length of JK and JL. What would you do than?(3 votes)
- Well, in this case, both JK and KL are equal, KL would just be JK. But if they weren't, it's just basic subtraction. You subtract the one side you know (JK) from the total length (JL) and get the remaining side (KL).(2 votes)

- Why did you subtract -7x+8 from 8x-8 ..?(1 vote)
- He knew that 8x-8=7x-6 and he was trying to put all the variables on one side and all the numbers on another side so that he could get x and solve for the total segment(6 votes)

- What if we have the 2 sublengths but we need to find out IF the middle point is actually the midpoint?(2 votes)
- if the three points are in the same line AND the sublengths are equal, it's indeed the midpoint(3 votes)

- How do you change the language of the captions?(3 votes)
- This is my first question, and it's completely out of curiosity. Is there a real world situation where someone would be provided with a line segment like the one above bisected equally in half, where someone provides two completely different algebraic values for the line? If someone's measuring a line and then dividing that measurement equally, why not just use the same algebraic measure for both halves of the line?(3 votes)
- I suppose you can think of it that way, but if you have two people measuring the segment half, then they might come up with two different algebraic measures. then, in that case, this process would come in handy!(0 votes)

## Video transcript

We're told that K is the
midpoint of segment, JL. So that tells us
that segment JK is going to be congruent
to segment KL, that they're going to be
the exact same length. And they tell us that segment
JK is equal to 8x minus 8. So this distance right over
here is equal to 8x minus 8. And then they tell
us that segment KL is equal to 7x minus 6, that its
length is equal to 7x minus 6. So this length right
over here is 7x minus 6. Because K is the
midpoint, we know that this length must
be equal to this length. So to find JL, we just need
to find the whole length. We defined what x is. If we know what x
is, then we're going to know what this length
is and what this length is. And we could either
double one of them or add them together to find the
length of the entire segment. So first, let's figure out x. And the best way to figure
out x is based on the fact that we know that 8x minus
8 is equal to 7x minus 6. And I just want to reemphasize,
how do we know that? Well, they told us that
K is the midpoint of JL. This is the midpoint,
which tells us that this distance is equal to
this distance, or 8x minus 8 is equal to 7x minus 6. Now, to figure out
x, we just have to do a little bit of algebra. So let's see what we can
do to simplify things. So if we want to get all
the x terms on one side-- let's say we're going to get
it on the left side-- we could subtract 7x from both sides. So let's do that. Let's subtract 7x
from both sides. And if we want to get all of
the constant terms on one side, well, let's add 8
to both sides so that we don't have this
negative 8 right over here. So let's add 8 to
both sides, and let's see what we are left with. On the left-hand side, you
don't have an 8 anymore, and 8x minus 7x is just an x. And that's going to be equal
to-- on the right-hand side, you don't have any 7x's anymore. And negative 6 plus 8 is just 2. So we get x is equal to 2. But we're not done yet. They didn't say solve for x. They said find JL. So JL is just going to
be the sum of JK and KL, or since K is the midpoint,
it would just be double JK or double KL. So let's figure out either way. So now we can figure out that
JK, the length of segment JK, is equal to 8 times 2 minus 8. We know that x is 2 now. So this is equal to--
well, this is 16 minus 8. This right over here
is just equal to 8. And if we wanted
to figure out JL, we know that this is halfway,
so that this must be 8 as well. And the length of the entire
thing, the length of JL must be 16. And if you wanted to spend
extra time to make sure that all of the
math is consistent, you could put 2 into
this right over here. And 7 times 2 is
14 minus 6 is 8. So, once again, you can
verify from another direction that this length of
segment KL is also 8. 8 plus 8 is 16.