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February 4th, 2010, 04:10 PM  #1 
Newbie Joined: Feb 2010 Posts: 3 Thanks: 0  Linear Systems of Equations: Substition Method
Why is that everytime I solve x in terms of y and substitute, I get a different answer (wrong)? I am trying to understand the intuition behind it.... ex: y=155x y=255x parallel lines means no unique solution substitute y in terms of x SUBSTITUTION 1: y=155x (155x)=255x 0=0 substitute x in terms of y SUBSTITUTION 2: y=155x therefore,5x=15y y=25(15y) 2y=10 y=5 Makes no sense??? 
February 4th, 2010, 06:42 PM  #2 
Newbie Joined: Feb 2010 Posts: 3 Thanks: 0  Re: Linear Systems of Equations: Substition Method
lol anyone???

February 5th, 2010, 01:39 PM  #3 
Global Moderator Joined: May 2007 Posts: 6,730 Thanks: 689  Re: Linear Systems of Equations: Substition Method
The particular pair of equations you chose has no solution. You can easily see that by subtracting one from the other and get as an equation 15=25.

February 7th, 2010, 04:57 AM  #4 
Newbie Joined: Feb 2010 Posts: 3 Thanks: 0  Re: Linear Systems of Equations: Substition Method
yes i am aware of this.... and you can prove it using substition... but my question is why is it that you can only solve systems of linear equations by substituing y in terms of x, and not x in terms of y? substitute y in terms of x SUBSTITUTION 1: y=155x (155x)=255x 0=0 substitute x in terms of y SUBSTITUTION 2: y=155x therefore,5x=15y y=25(15y) 2y=10 y=5 
February 7th, 2010, 05:57 AM  #5 
Senior Member Joined: May 2008 From: York, UK Posts: 1,300 Thanks: 0  Re: Linear Systems of Equations: Substition Method
As long as there is a solution, you can always solve for either variable first. You have made an error in your second substitution: which is clearly false 

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equations, linear, method, substition, systems 
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