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 June 30th, 2012, 05:04 AM #1 Senior Member   Joined: Sep 2011 Posts: 395 Thanks: 0 Simultaneous Equations Having a bit of trouble with this one. Can anyone help? Many thanks. Q. Solve the 3 unknowns: $x+y+2z=3$...1 $4x+2y+z=13$...2 $2x+y-2z=9$...3 Attempt: $x+y+2z=3$...1 $4x+2y+z=13$(mult. by -2)...2 $x+y+2z=3$...1 $-8x-4y-2z=-26$...2 => Adding 1 & 2 yields... $-7x-3y=-23$...4 $4x+2y+z=13$(mult. by 2)...2 $2x+y-2z=9$...3 $8x+4y+2z=26$...2 $2x+y-2z=9$...3 => Adding 2 & 3 yields...$10x+5y=35$...5 $-7x-3y=-23$(mult. by -5)...4 $-10x-5y=-35$(mult. by 3)...5 $35x+15y=115$...4 $-30x-15y=-75$...5 => Adding 4 & 5 yields...$5x=40$ => $x=8$ Ans: (From text book x = 2, y = 3, z = -1
June 30th, 2012, 05:20 AM   #2
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Re: Simultaneous Equations

Quote:
 Originally Posted by bilano99 Q. Solve the 3 unknowns: $x+y+2z=3$...1 $4x+2y+z=13$...2 $2x+y-2z=9$...3
Try again; start by adding together 1 and 3 to get right away: 3x + 2y = 12

June 30th, 2012, 05:39 AM   #3
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Re: Simultaneous Equations

Quote:
 Originally Posted by bilano99 $-7x-3y=-23$(mult. by -5)...4 $-10x-5y=-35$(mult. by 3)...5 $35x+15y=115$...4 $-30x-15y=-75$...5 => Adding 4 & 5 yields...$5x=40$ => $x=8$
Careful -35 * 3 = -105, not -75

 June 30th, 2012, 05:48 AM #4 Senior Member   Joined: Sep 2011 Posts: 395 Thanks: 0 Re: Simultaneous Equations D'oh. Silly error. Thank you.

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