# slopes

• Oct 2nd 2008, 08:08 AM
Geo
slopes
Ok I think I've worked this out incorrectly so if someone could point me in the right direction that would be great.....

The question is find the equation of a line that passes through (-1,5) and (2,-4)
I know that
y=mx+c to find M = change in y value divide by change in x value=

2- (-1)/5-(-4) = 3/9 =1/3

so y = 1/3x=c

so 5=1/3(-1)+c
5= -1/3+c
5+-1/3 = c
4 2/3 = c so
y= 1/3x+ 14/3

You see why I need help right?
• Oct 2nd 2008, 09:23 AM
masters
Quote:

Originally Posted by Geo
Ok I think I've worked this out incorrectly so if someone could point me in the right direction that would be great.....

The question is find the equation of a line that passes through (-1,5) and (2,-4)
I know that
y=mx+c to find M = change in y value divide by change in x value=

2- (-1)/5-(-4) = 3/9 =1/3 Your slope is calculated incorrectly. Look at my slope formula below.

so y = 1/3x=c

so 5=1/3(-1)+c
5= -1/3+c
5+-1/3 = c
4 2/3 = c so
y= 1/3x+ 14/3

You see why I need help right?

First find the slope using (-1, 5) and (2, -4):

$\displaystyle m=\frac{y_2-y_1}{x_2-x_1}$

$\displaystyle m=\frac{-4-5}{2-(-1)}$

$\displaystyle m=\frac{-9}{3}=-3$

Now, use y=mx + b to find the y-intercept (b). We'll use (x, y) = (-1, 5) for our point and m = -3.

$\displaystyle y=mx+b$

$\displaystyle 5=-3(-1)+b$

$\displaystyle 2 = b$

Finally, putting it all together with m=-3 and y-int = 2, we have:

$\displaystyle y=-3x+2$
• Oct 2nd 2008, 09:36 AM
Geo
Right had them round the wrong way. That clears it up. Your a star!!