Maths Help - Simultaneous Equations

Captain-Fungi · 12 Oct 2006 at 19:00

Guys, I am really stuck on this one question for solving the simultaneous equations; I know there are some maths very clever people on here so I thought I would ask.

x-5y=7
3x+y=5

I have been trying this one question for going on two hours now and I'm starting to lose my sanity.

I thought that if I could rearrange the equations so that Y was the subject I could plot them on my graphics calculator and see the co-ordinate where the lines crossed and hopefully get the answer

I started off by rearranging 3x+y=5 and came up with y=5+3x

But I am stuck with rearranging the other equation and can only come up with the answer 5y=7-x Should it be y=(7-x)/5 ??

I am not sure, any help is greatly appreciated as its really getting to me now.

I dont really want the answer (yet

) just some pointers would be a real help.

Thanks

/CF

Melm0 · 12 Oct 2006 at 19:05

I don't know if this helps but you could try subbing in y=5+3x into x-5y=7 so you get x-5(5+3x) and then work out x? Then do the business with y.

bellamy · 12 Oct 2006 at 19:07

3x+y=5 becomes y=5-3x not the way you got
x-5y=7 becomes y=(x-7)/5

try that

GeForce · 12 Oct 2006 at 19:10

================
WARNING: ANSWER IN THIS POST

I can't think of any other way to explain it, sorry. Don't read it if you don't want to

================

x-5y=7 - equation 1
3x+y=5 - equation 2

Multiply eq1 by 3: 3x-15y=21

3x-15y=21
3x+y=5

Take 1) away from 2).

3x-3x = 0.
-15 - 1 = -16.
21 - 5 = 16.

-16y = 16
16y = -16
y = -1

put that back into 2)

3x - 1 = 5
3x = 6
x = 2

=========
Answer: x = 2, y= -1
=========

Jon :cool:

SiriusB · 12 Oct 2006 at 19:10

http://www.bbc.co.uk/schools/gcsebitesize/maths/algebraih/simultaneousrev2.shtml

Easy when you know how

SiriusB

Melm0 · 12 Oct 2006 at 19:11

He said he didn't want the answer :rolleyes:

Captain-Fungi · 12 Oct 2006 at 19:16

Thanks guys, for all the tips and to Geforce for taking the time to work through the answer.

I now understand it well enough to do myself.

Thanks

PinkPig · 12 Oct 2006 at 19:33

Captain-Fungi said:
I now understand it well enough to do myself.

Thanks

Also make sure you understand why everything you said in your original post was wrong - and that spending 2 hours on that sort of question is just plain stupid, you should have gone for advice from someone sooner than that. I can only assume you actually spent 2 hours staring at it and kidding yourself that you were working.

SpeedFreak · 12 Oct 2006 at 21:18

I know you said you understand it, but I'll give you my help anyhow.

You always need to remove one of the unknown variables to leave just numerics and either x or y.
Doing this involves scaling one or both of the equations and then adding or subtracting the results.

eg/if there is a 5y in one and a y in the other, you need to multiply that equation by 5 and then subtract the reuslts leaving X and numerics.

Biohazard · 12 Oct 2006 at 21:24

I'm pants at maths.. really.