Question

How do I know which is right-
y2-y1 over x2-x1 or
y1-y2 over x1-x2
People say it's the same on yahoo answers but they get different answers a lot. HELP
It's different.
Let's say (-2,8) and (2,-1).
First way: -1 - 8 = -9
2 - (-2) = 0
So... -9/0
Second way: 8- (-1) = 9
-2 - 2 = 0
So... 9/0

See it's like this all the time. In this example they are the same because it is undefined, but in other examples they are regular numbers that are negatives.

Answer 1

Its the same, but in different order.

Answer 2

either one works... so long as your going the same way.

point 1 to point 2 or point 2 to point 1

Answer 3

check it out...

\[ \frac{y_{2} - y_{1}}{x_{2} - x_{1}}=\frac{-(y_{1} - y_{2})}{-(x_{1} - x_{2})}=\frac{y_{1} - y_{2}}{x_{1} - x_{2}}\]

Answer 4

If you choose a y1, the x1 better be from the same point. As long as you make x1,y1 the same point and x2,y2 the same point, it really doesnt matter

Ex. (3, -4) and (-5,2)

So lets make the (-5,2) my point 2
\[\frac{ 2-(-4) }{ -5-3 }=\frac{ -3 }{ 4 } \]

Now lets do it the other way, Ill have (3,-4) be my point two:

\[\frac{ -4-2 }{ 3-(-5) }=\frac{ -3 }{ 4 }\]

Answer 5

yes... check your arithmetic @Mia_Rachel

Answer 6

What did I do wrong?

Answer 7

I think you guys scared her

Answer 8

2-(-2) = 2 + 2 because subtraction is addition of the opposite

Answer 9

Psymon and pg scaring users with their math? Not surprising

Answer 10

also, -2 - 2 = -2 +(-2) = -4

Answer 11

what? when did math get scary?

Answer 12

\[\frac{ -1-8 }{ 2-(-2) }=\frac{ -9 }{ 4 } \]

\[\frac{ 8-(-1) }{ -2-2 }=\frac{ -9 }{ 4 } \]

Answer 13

challenging, frustrating, down right nasty... but not scary!

Answer 14

I think for a lot of people it gets scary when there are no numbers. Which is kind of ironic.

Answer 15

letters are so much easier to deal with...

Answer 16

I'm sorry it's not (2,-1) it's (-2,-1).

Answer 17

in that case you're dividing by 0, either way you do it. since division by 0 leads to an undefined quotient, the slope is undefined in both cases.

you'll learn more about this in calculus when you actually look closer at division by 0.

Answer 18

Well, either way, even though the numbers I used were not the correct numbers, can you see how you still get the same answer if you do the math correct?

Answer 19

Can you do it with the same numbers too? I like to see examples. :/

Answer 20

I gave an example way up above already using numbers I invented.

Answer 21

With the numbers I use please. (-2,8) (-2,-1)

Answer 22

what you did is correct... you get 2 seemingly different answers, but they're not.

in both cases you're dividing by 0 which is undefined... meaning both ways actually give you an undefined slope.

if you examine the numbers, they have the same x coordinate meaning you have a vertical line.
what's the slope of a vertical line?

Answer 23

undefined

Answer 24

Honestly, your example doesnt perfectly show how the signs end up being the same. If one of those -2's were any different number, you would come out with the same answer and you would not have to worry about having the incorrect sign. In your case it's just undefined no matter what, which means a vertical line.

Answer 25

Thanks! Sorry, I just got back to school and I can't remember anything in my math class (Alg II/Trig) and I'm in 9th grade.

Answer 26

But can someone solve it for me? Apparently the answer is x=-2 (back of textbook) but can someone show me step by step how that happens?

Answer 27

exactly... and what is \[\frac{9}{0} \text{ ?}\]
what about
\[-\frac{9}{0} \text{ ?}\]

Answer 28

they're both undefined, right?

like i said, you'll look more closely at this in calculus.

Answer 29

Someone please show me step by step :(

Answer 30

for the whole problem

Answer 31

you already did it Mia... what is giving you trouble?

Answer 32

I mean make up the linear equation with that. I still need to know b (y=mx+b).

Answer 33

check it out... you have a vertical line. they have a special form (as does a horzontal line).

for a vertical line, it's x = some number. in your case, x = -2 because no matter what y is, x is always -2.

for a horizontal line, the equation will always be y = some number, for example, y = 5. no matter what x is, y will always be 5

Answer 34

I know that, but where did you get the -2?

Answer 35

from the points you gave... (-2, 8) and (-2, -1) see, x is always -2.

Answer 36

OH DUHHHHHHH

Answer 37

lol

Answer 38

But what's the mathematical way you find that?

Answer 39

when you find that the slope is undefined, that's when you know you have a vertical line. then you know the eqution of the line willl be in that form.

likewise, when you get a slope of 0, you know you have a horizontal line and that the equation will be y = some number and some number will be the y coordinate of any of your points.

Answer 40

You are a SAVIOR. THANKS I'm done with all of my questions xD

Answer 41

thanks for sticking with it and making sure you understand! you're on your way to being an excellent mathematician!!!

Answer 42

Ahaha thanks, I'm stubborn (:

Answer 43

an excellent quality for anyone who wants to learn and understand ; )