# Thread: equation of straight line with co ordinates parallel to...

1. ## equation of straight line with co ordinates parallel to...

hi,

i need to know how to answer this question:

The points A,B and C has co ordinates, (-3,0) , (5,-2) and (4,1)
find the equation of a straight line through C and parallel to AB. Write answer in form of ax+by+c=0

im not too sure how to start this one however i can do equation of a straight line with 2 co ordinates, so i require some help please. could you please exaplin what you do please so i gain an understanding?

thanks!

2. ## Re: equation of straight line with co ordinates parallel to...

Originally Posted by andyboy179
The points A,B and C has co ordinates, (-3,0) , (5,-2) and (4,1) find the equation of a straight line through C and parallel to AB. Write answer in form of ax+by+c=0
Parallel lines have the same slope.
If $\displaystyle m\text{ is the slope of }\overleftrightarrow {AB}$ then your answer is $\displaystyle y-1=m(x-4)$.

3. ## Re: equation of straight line with co ordinates parallel to...

thanks for answering! how did you get that? would i do:

0=-3m+c
-2=5m+c

minus the top number from the bottom to get :

-2=2m+c
-2/2=-1 M=-1?

4. ## Re: equation of straight line with co ordinates parallel to...

Originally Posted by andyboy179
would i do:
0=-3m+c
-2=5m+c
minus the top number from the bottom to get :
-2=2m+c
-2/2=-1 M=-1?
None of that is correct.
If you do not know how to find the slope given two points then you cannot do this problem. So find how to do slope if you do not know. It is in your text/notes.

5. ## Re: equation of straight line with co ordinates parallel to...

oh so:

0--2/ -3-5= 2/-8?

6. ## Re: equation of straight line with co ordinates parallel to...

Originally Posted by andyboy179
oh so:
0--2/ -3-5= 2/-8?
So $\displaystyle m=\frac{-1}{4}$. Use in the point-slope form in reply #2.

7. ## Re: equation of straight line with co ordinates parallel to...

unsure on this next part, i dont understand how it works

8. ## Re: equation of straight line with co ordinates parallel to...

Originally Posted by andyboy179
unsure on this next part, i dont understand how it works
then research how it works ...

Straight-Line Equations: Point-Slope Form

9. ## Re: equation of straight line with co ordinates parallel to...

ohh, so would it be:
y-1=m(x-4)
y-1= -0.25x+0.1
y=-0.25x+0.1+1
y=-0.25+1.1??

10. ## Re: equation of straight line with co ordinates parallel to...

Originally Posted by andyboy179
ohh, so would it be:
y-1=m(x-4)
y-1= -0.25x+0.1 ???
y=-0.25x+0.1+1
y=-0.25+1.1??
$\displaystyle y-1 = -\frac{1}{4}(x - 1)$

multiply by 4 ...

$\displaystyle 4y - 4 = -1(x-1)$

distribute ...

$\displaystyle 4y-4 = -x+1$

$\displaystyle x+4y-5 = 0$

... the final form as stated in the instructions.