Graphing Slope Intercept

Date: 02/13/99 at 11:25:33
From: jeremy alexander
Subject: Graphing slope when given in slope intercept form

When given the slope intercept form, how do you know if you rise or 
drop when looking at the slope? Also, do you find the y-intercept and 
go from there in looking for the slope? I looked at examples in my 
Algebra book and it gives me no explanation of how to do it.

Date: 02/17/99 at 18:24:54
From: Doctor Mike
Subject: Re: Graphing slope when given in slope intercept form

If the slope is greater than zero, the line goes up as you follow it 
going from left to right. If the slope is negative, the line goes down 
as you go from left to right. If the slope is zero, the line is 
horizontal. 
   
For the 2nd question, I am not sure which situation you are dealing 
with. If you know the y-intercept, that gives you a point (0,B) on 
the line in question. If you also know another point (C,D) on the line, 
then you can use those 2 points to find the slope. In general, if you 
know 2 points (A,B) and (C,D) on the line, then the slope is
   
           D - B 
          -------
           C - A 
   
This can be remembered as 'Change in y over Change in x'. Notice that 
I subtracted the coordinates of (A,B) from the coordinates of (C,D).  
You might ask what would have happened if I had done it the other way 
and subtracted the coordinates of (C,D) from the coordinates of (A,B) 
giving
   
           B - D 
          -------
           A - C 
  
The result would be the same. Just be sure you use the same order for 
both the numerator and denominator.  
  
Example: Slope of line from (-1, 2) to (3, -4) is
   
           2 - (-4)       +6 
          ----------  =  ----  =  -1.5  
           -1 - (3)       -4   
   
and the slope is less than zero, so the line goes down to the right. 
If you do the slope using the 2 points in the other order, you will get 
(-6)/(+4), which also equals -1.5. 
   
I hope this helps.  

- Doctor Mike, The Math Forum
  http://mathforum.org/dr.math/