# Determining slope and Y-Int

• December 17th 2007, 03:12 PM
hemi
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!
• December 17th 2007, 03:57 PM
colby2152
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.
• December 17th 2007, 06:39 PM
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?
• December 17th 2007, 06:59 PM
colby2152
Quote:

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.
• December 17th 2007, 07:30 PM
hemi
Quote:

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... :)
• December 17th 2007, 08:55 PM
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 $\frac {y_2-y_1}{x_2-x_1}$

$\frac {0-12}{6-0} = -2$
• December 18th 2007, 07:45 AM
hemi
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!
• December 18th 2007, 07:52 AM
colby2152
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!
• December 18th 2007, 08:00 AM
hemi
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?
• December 18th 2007, 05:25 PM
Quote:

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.

Quote:

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