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  • Subject: Maths time Flood!
Subject: Maths time Flood!

Pic of my problem

I'm trying to work through a test I've done to practice and am going through the mark scheme to look at where I went wrong. At this point I'm looking at the equation of AB (and I have that right) but I can't see how they got form that equation into the correct form ax + by + c = 0.

For reference the question is: Find the equation of the line joining A(7,4) and B(2,0) giving the answer in the form ax + by + c = 0 where a, b and c are integers.

Thanks for the help Flood :D

  • 12.03.2012 9:26 AM PDT

So first we want to find the slope of the line.

Rise / Run = (4 - 0)/(7 - 2) = 4/5

So our line is going to be some form of y = 4/5x + B

(2, 0) is a point on the line, so lets plug in 2 for x and 0 for y and solve for B.

0 = (4/5)(2) + B
B = -8/5

So the line is y = 4/5x - 8/5, which can be turned into y - 4/5x + 8/5 = 0.

Multiplying by 5 naturally gets you a cleaner answer, but this technically works ;)

[Edited on 12.03.2012 9:31 AM PST]

  • 12.03.2012 9:30 AM PDT


Posted by: Hylebos
So first we want to find the slope of the line.

Rise / Run = (4 - 0)/(7 - 2) = 4/5

So our line is going to be some form of y = 4/5x + B

(2, 0) is a point on the line, so lets plug in 2 for x and 0 for y and solve for B.

0 = 4/5(2) + B
B = -8/5

So the line is y = 4/5x - 8/5, which can be turned into y - 4/5x + 8/5 = 0.
Except the answer is 4x-5y-8=0

You need to use a form of the equation y - y1 = m(x - x1) I believe

  • 12.03.2012 9:31 AM PDT

Posted by: DarkBen64
Except the answer is 4x-5y-8=0

You need to use a form of the equation y - y1 = m(x - x1) I believe
Multiply both sides of my answer by 5 and you'll see that they are equivilant.

[Edited on 12.03.2012 9:33 AM PST]

  • 12.03.2012 9:32 AM PDT


Posted by: Hylebos
Posted by: DarkBen64
Except the answer is 4x-5y-8=0

You need to use a form of the equation y - y1 = m(x - x1) I believe
Multiply both sides of my answer by 5 and you'll see that they are equivilant.
I"m not seeing it. You have +8, not -8.

Edit: wait, no, I see it. I'm being thick.

They make you use y = mx + c even though the other equ has been taught because it' s better. damn.

[Edited on 12.03.2012 9:37 AM PST]

  • 12.03.2012 9:35 AM PDT

Fair play

Man, I really hate math. Why would you make a math thread?

  • 12.03.2012 9:36 AM PDT

Posted by: DarkBen64
Posted by: Hylebos
Posted by: DarkBen64
Except the answer is 4x-5y-8=0

You need to use a form of the equation y - y1 = m(x - x1) I believe
Multiply both sides of my answer by 5 and you'll see that they are equivilant.
I"m not seeing it. You have +8, not -8.
Let me double check my work then. I was hastily solving the problem as I had about a minute of power left on my laptop and wanted to get to a power source. I'm plugged in now so I should have time.

AH.

y - (4/5)x + (8/5) = 0
(-5)(y - (4/5)x + (8/5)) = (-5)(0)
4x - 5y - 8 = 0
There we go. Equivilant answers are equivilant.

[Edited on 12.03.2012 9:43 AM PST]

  • 12.03.2012 9:37 AM PDT