# slope point equation

Printable View

• Sep 21st 2010, 08:18 PM
mrnemo
slope point equation
This should be easy. This is first time I'm doing these in over a decade.
Write the equation of the line that goes through point P and has slope m.
P(-5,2), m = -1

Here is what I did:

Y – y1 = m( x – x1)
Y – 2 = -1(x – (-5))
Y – 2 = -x + 5
Y = -x + 7
X + y = 7
Here is what the answer is supposed to be:
x + y = -3

how did I screw this up? Thank you.
• Sep 21st 2010, 08:28 PM
ragnar
There's a mistake going from line 2 to 3 on the right-hand side. The x - (-5) becomes x + 5, but when you distribute the -1, you get -x - 5.

I find that there's an easier way to do these sorts of problems:

We know the slope is -1 so let's pretend the equation of the line is y = -x. We want it to run through (-5, 2), so we plug in -5 for x. y = 5 then. So we are 3 units above our goal, which is 2. So we need to come down 3. Thus, the equation is actually y = -x - 3.
• Sep 21st 2010, 08:32 PM
Educated
EDIT: nevermind
• Sep 21st 2010, 08:34 PM
mrnemo
Thanks for the help