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January 6th, 2017, 09:15 PM  #1 
Member Joined: Jan 2017 From: US Posts: 86 Thanks: 5  Which Method Should I use to Solve this System of Equations?
x + y = 4 y = 3x I am not sure which method I should use for this system of equations. I could use the addition method, but there are't any numbers aside from 4 in the first equation, and the same would happen when using the elimination method, so how would that work? I can use either the addition method, the elimination method, the substitution method or the graphing method. Just looking at this, I have determined the ordered pair (1,3) to be the solution, but that is without using a particular "method" which is what I am supposed to do. Any help would be appreciated! Thanks Last edited by Indigo28; January 6th, 2017 at 09:22 PM. 
January 6th, 2017, 09:26 PM  #2 
Senior Member Joined: Feb 2016 From: Australia Posts: 1,170 Thanks: 384 Math Focus: Yet to find out. 
Substitution looks easiest, but maybe you solved it intuitively, without really thinking about which method you used This can be a good sign of your understanding! It's given that y = 3x in the second equation, so just substitute your expression for y in the first equation; x + 3x = 4 4x = 4 x = 1. Now you know what x is, substitute it back into the second equation! y = 3(1) = 3. Looks to me like your answer is correct. 
January 6th, 2017, 09:30 PM  #3  
Member Joined: Jan 2017 From: US Posts: 86 Thanks: 5  Quote:
Wow that looks a lot more simple than I made it out to be lol. Thank you!  
January 6th, 2017, 09:32 PM  #4 
Senior Member Joined: Feb 2016 From: Australia Posts: 1,170 Thanks: 384 Math Focus: Yet to find out.  
January 7th, 2017, 05:36 AM  #5 
Global Moderator Joined: Dec 2006 Posts: 17,177 Thanks: 1285 
x + y = 4 3x  y = 0 Adding those eliminates y, giving 4x = 4, so x = 1. It's now possible to find y by substituting 1 for x in either equation. Alternatively, multiplying the first equation by 3 gives 3x + 3y = 12. Subtracting the second equation from that gives 4y = 12, so y =3. 

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