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November 19th, 2011, 07:31 PM  #1 
Senior Member Joined: Nov 2011 Posts: 101 Thanks: 0  system of equations (find x,y,z) questions
Hi. I have problem with these two questions. 1. 3x+2yz=2 4x5y+2z=1 7x+y2z=3 So I did use 1st and 2nd equation to get 10xy=3 and 1st and 3rd to get x+3y=1. If I use these two equations I get y=7/31 and when I plug this in I get x=10/31. Now I plug those two for x and y on 1st equation I get z=18/31. So solution set for this equation comes out (7/31,10/31,18/31) for x,y,z order. When I plug these 3 #s for x,y,z for 1st and 2nd equations I get 2 and 1 which is correct answer. The problem is when I plug them in 3rd equation, I don't get 3 but I get 3.64.... is my answer wrong or did I make some mistake? plz help. Here is 2nd problem I am having trouble with. I'm supposed to find x, y, z and z is arbitrary # 2. 5xy+3z=7 2x+4y+9z=11 So if z is arbitrary, I solve for y, I get y=(39z/22)+(69/22). If I plug that in y section, I get x=(21z/22)(17/22). So my answer set is ( (21z/22)(17/22), (39z/22)+(69/22), z(arbitrary) ). Is this right answer? I need help. 
November 19th, 2011, 07:56 PM  #2  
Math Team Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 11,376 Thanks: 730  Re: system of equations (find x,y,z) questions Quote:
Did you not notice a shortcut: adding equations 2 and 3 right away?  
November 19th, 2011, 08:04 PM  #3  
Math Team Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 11,376 Thanks: 730  Re: system of equations (find x,y,z) questions Quote:
 
November 19th, 2011, 08:18 PM  #4 
Senior Member Joined: Nov 2011 Posts: 101 Thanks: 0  Re: system of equations (find x,y,z) questions
Ok. thx. I got first one's correct answer ( (10/29), (13/29), (2/29) ) I get correct answer with these x,y,z #s. As for second one, are you sure I am right? These #s just don't look pretty enough to me to be an answer. I remember my instructor saying, I won't get pretty # answers like I did in algebra 1, 2 since this is college algebra but even so, it looks ugly. 
November 20th, 2011, 12:06 AM  #5 
Global Moderator Joined: Dec 2006 Posts: 18,432 Thanks: 1462 
The answers for the second problem may be inelegant, but they are correct.


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