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## #1 2005-07-25 12:29:53

schwebba
Member
Registered: 2005-07-25
Posts: 2

### Substitution Method (where two lines cross)

I got some trouble with finding out how to calculate this..

Question: Where do these lines cross eachother x + 3y +3=0 and x - 3y +2 =0

First i tried to solve y and got this far

y = -1/3x - 1 = 1/3x+2/3          (divided all with 3)

but im not even sure if that is right. can anyone please explain how i can do this?

Last edited by schwebba (2005-07-25 12:32:03)

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## #2 2005-07-25 16:26:05

ganesh
Registered: 2005-06-28
Posts: 25,023

### Re: Substitution Method (where two lines cross)

Solving the two equations, you get
x = -5/2, y = -1/6
Hence, the two lines cross at
(-5/2, -1/6)

How the two equations are solved:-
First equation can be written as                x+3y = -3
The second equation can be written as     x - 3y = -2
Adding the two, we get                           2x = -5, or x = -5/2
Substituting this value in equation (1)     -5/2 + 3y = -3
3y = -3 + 5/2
3y = -1/2
y = -1/6

It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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## #3 2005-07-25 19:10:46

MathsIsFun
Registered: 2005-01-21
Posts: 7,685

### Re: Substitution Method (where two lines cross)

Neatly solved!

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..."  - Leon M. Lederman

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## #4 2005-07-25 22:47:52

schwebba
Member
Registered: 2005-07-25
Posts: 2

### Re: Substitution Method (where two lines cross)

wow thanks i think i get it now. you sure cleared things up a bit

gonna practise some more on this..

thanks a lot for that nice solution

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## #5 2005-07-27 21:09:04

MathsIsFun
Registered: 2005-01-21
Posts: 7,685

### Re: Substitution Method (where two lines cross)

And for all those avid graphers out there, I have a little tool to figure out lines from points here: http://www.mathsisfun.com/straight-line … ulate.html

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..."  - Leon M. Lederman

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