Substitution Method

[Stine]

Solve this system of linear equations by the substitution method:

2x+y+4
x-4y+1

What I have came up with so far is :
x-4(4-2x)=1
x-16+8x=1+16
(9x/9)=(17/9)

plug it in to 2(17/9) +y=4

Where did I go wrong????? This is really ugly and not workign for me when I finish in the end.

Thank you for your help-

 

[jwpaine]

{2x+y+4
{x-4y+1

gives you

{2x+y = -4
{x - 4y = -1

use the top or bottom for substitution. I'll use the top, and solve for x first.

2x-2x+y=-4-2x
y = -4-2x

plug in y=(-4-2x) into either equation, and solve for x. I'll use the bottom one.

x - 4(-4-2x) = -1
x +16+8x = -1
9x + 16 = -1
9x = -17
x = (-17/9)

Can you see where you made a mistake?

Now plugin x = (-17/9) into x in either equation, and solve for y


Hope this helps.

 

[Stine]

Thank you

I am sorry I thought I read over my problem more thorough to make sure everything was right- Thank you for helping me out with this (it being set up incorrectly and you correcting it for me) I will do my best to be more thorough before posting! Thanks again!

Stine

 

[Stine]

2x + y = 4
x - 4y = 1

plug 2(17/9) + y= 4
=-3(7/9) + y= 4

plug (17/9) - 4y= 1
= -4y=1-(17/9) and divide that by -4.

Am I doing alright so far? This is really ugly and doesn't seem correct to me...

 

[jwpaine]

I thought we /just/ went over this, no?

That is the exact same problem as I showed above.

PLEASE review my steps and explination that I took time to show.

 

[Stine]

Wow what a response- :shock: Ok We did go over the problem. I saw that the first time. It was not overlooked- I am just saying that I tried it and what I was coming up with didn't seem correct to me (or shall i say it was really ugly) and just wanted to know if I was doing it correct after I went through all the steps myself again.

THANK YOU for YOUR TIME.

 

[Denis]

2x + y = 4
x - 4y = 1

Stine, you must be tired or something!
Those equations were originally:
2x + y = -4
x - 4y = -1

 

[skeeter]

Solve this system of linear equations by the substitution method:

2x+y+4
x-4y+1

this is what was originally posted ... neither is an equation. Maybe the last (+) sign in each line was meant to be typed as (=) ???