# Thread: Determining slope and Y-Int

1. ## Determining slope and Y-Int

So... I've been away from the math world for sometime and I need some help here.

I have this graph:

And I need to determine the slope and y intercept on it, can someone please tell me how I would do that in good detail so I can remember this stuff?

Edit: Hahha I think I remembered, for slope would this be right...
m= (6, 3) / (8, 2) = 0.5

So would 0.5 be the slope if I used those two points?

Still forgot how to find Yint though haha.

Thanks!

2. Keep in mind the slope is actually -0.5. Slope is rise over run (from left to right). It rises negative 1 or drops 1 and runs 2.

$m=\frac{-1}{2}$

There are a number of ways you can find the y-intercept. Let's stick with the slope intercept form.

$y=mx+b$
$3=-.0.5(6)+b$
$b=6$

I plugged in the coordinates (6, 3) and used the slope -0.5 that we figured out already. If those coordinates are correct, then your graph is wrong.

3. So... how is my graph wrong? Lets try it out with different coordinates...

8,2
4,4 are the coordinates

so we write...
8-2=6
4-4=0

m = 6 ?

then what?

What coordinates would make this graph work?

4. Originally Posted by hemi
So... how is my graph wrong? Lets try it out with different coordinates...

8,2
4,4 are the coordinates

so we write...
8-2=6
4-4=0

m = 6 ?

then what?

What coordinates would make this graph work?
You gave coordinates (6, 3)... If you meant (3, 6) [which would match your graph], then my answer is incorrect. In that case, the y-intercept would be 12. You can even see that in the graph you gave.

5. Originally Posted by colby2152
You gave coordinates (6, 3)... If you meant (3, 6) [which would match your graph], then my answer is incorrect. In that case, the y-intercept would be 12. You can even see that in the graph you gave.
Ok I see. So lets try this again here...

M = 3, 6/ 2, 8

= 3/6 = 2

So slope = 2

And Y-Int:
y=mx+b

6 = 2(3) + b
b = ?

What would b equal and are my calculations all correct in and intact with this graph I have here?

Thanks a lot for your help btw...

6. Looking at your graph, it is clear that it passes through 2 points: (0,12) and (6,0). The y-intercept is the point where x = 0, in this case you can just read it off the y-axis as 12.

the gradient is given by the formula $\frac {rise}{run}$or if you find it clearer this way $\frac {y_2-y_1}{x_2-x_1}$

$\frac {0-12}{6-0} = -2$

7. thanks a lot bro!

But I like the y = mx+b equation....

So how would I plug in the coordinates you gave me into that and get the Y-Int of 0?

Thank you SO much for helping!

8. You know the slope is $m=-2$ and you have a point on the line. Use the (x, y) coordinates of a point on the line, plug the points and the slope into the equation, and chug!

9. Thing is, I don't know how to "chug".. Would you mind explaining that to me in the simplest terms possible?
Sorry for being a pest, really.

One more question though, say I have the graph...

and I pick the points 10, 10 and 8, 8 -- I would then write:
m = 8 - 8/10-10 = 0 ?
Is that possible? If not what points would make this graph work?

10. thanks a lot bro!

But I like the y = mx+b equation....

So how would I plug in the coordinates you gave me into that and get the Y-Int of 0?

Thank you SO much for helping!
The y intercept is the y value where x = 0. So just substitute y = m0+b
so y=b.

and I pick the points 10, 10 and 8, 8 -- I would then write:
m = 8 - 8/10-10 = 0 ?
(x_1,y_1) = (10,10)
(x_2,y_2) = (8,8)

so
m=

so m = $\frac {8-10}{8-10}$
= $\frac {-2}{-2}$
=1