Solving Equations using the elimination method. Help!

Bob · 2012-07-04 07:21:17

Agreed.

But the equation you have ended up with is not the one you started with.

Compare

y = x + 2 (0,2) (1,3) (2,4) (3,5) (4,6) and so on.

x + y = 2 (0,2) (1,1) (2,0) (3,-1) (4,-2) and so on.

So x + y = 2 is NOT a correct re-arrangement of y = x + 2

Bob

SlowlyFading · 2012-07-04 07:24:46

Don't I need to change the first equation aswell then?

Bob · 2012-07-04 07:29:14

4 x + 3 y = -15

You have the right format for this.

Bob

SlowlyFading · 2012-07-04 07:35:53

I don't see any other way to make the variables line up besides making the second equation x + 2y = y and I'm not sure if that is even legal in math.

Bob · 2012-07-04 07:45:16

OK I'll show you.

You have y = x + 2.

You want the x term on the same side as the y term.

So subtract x from each side.

y = x + 2
-x -x
________________

-x + y = 2

Now they are lined up.

4x + 3y =-15

-x + y = 2

But the amounts of x and the amounts of y are not the same.

For elimination to work they must be the same.

So take the second equation and multiply each term by 4.

Then it will also start -4x .........

But what is the rest of this equation?

Bob

SlowlyFading · 2012-07-04 07:48:54

4x + y = 8 ?

Bob · 2012-07-04 07:51:24

multiply each term by 4.

You haven't multiplied every term by 4

Bob

SlowlyFading · 2012-07-04 07:56:49

4x + 4y = 8 ?

I'm getting kinda confused now..

Bob · 2012-07-04 07:58:03

That's closer. But why is the x term now positive?

Bob

SlowlyFading · 2012-07-04 08:00:58

Because multiplying cancled it out? Or is it supposed to be there?

Bob · 2012-07-04 08:07:19

The equation was -x + y = 2

Multiply by 4

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

-4x + 4y = 8

So now we have 4x in both equations.

4x + 3y = -15

-4x + 4y = 8
________________

This time the terms that are the same have opposite signs.

So don't subtract one from the other. Rather add them.

Post back what you now have.

Bob

SlowlyFading · 2012-07-04 08:13:26

x = 12
y = 48

SlowlyFading · 2012-07-04 08:15:37

Or y = 3 ?

Bob · 2012-07-04 08:16:29

???????

I was expecting an equation with the x terms cancelled so just an equation in y.

What equation did you get?

Bob

SlowlyFading · 2012-07-04 08:31:07

Is this the equation 0 + 7y = 23?

Bob · 2012-07-04 08:34:13

Well that's almost what I was expecting.

But -15 + 8 is not 23

Bob

SlowlyFading · 2012-07-04 08:36:51

Opps, it's -12

SlowlyFading · 2012-07-04 08:38:09

I thought I had to multiply it.

Bob · 2012-07-04 08:42:08

Not -12 either

-15 + 8

Bob

SlowlyFading · 2012-07-04 08:44:52

-7

Bob · 2012-07-04 08:49:57

so y = ?

Bob

Bob · 2012-07-04 08:57:45

I think you need to practise basic algebra.

here's a couple of useful links:

http://www.mathsisfun.com/algebra/introduction.html

and

http://www.mathsisfun.com/algebra/add-s … lance.html

Bob

SlowlyFading · 2012-07-04 09:07:07

x = 0
y = 0

Bob · 2012-07-04 09:29:30

???????

You had 0 + 7y = 23 but that was because you did 15 + 8 instead of -15 + 8

So you corrected it to 0 + 7y = -7

where did y = 0 come from?

think about it.

If x = 0 and y = 0 was the answer you'd have

y = x + 2 ...... 0 = 0 + 2
4x +3y = -15 ........ 4 times 0 + 3 times 0 = -15.

Whenever you get an answer think about whether it seems reasonable.

So please try to solve 0 + 7y = -7.

Bob

SlowlyFading · 2012-07-04 10:31:59

I thought to solve that I had to divide the 7y by 0 and the -7 by 0?
If that's not the case, i don't know. Like I said, I'm only just learning the steps to this type of math.

Math Is Fun Forum

#26 2012-07-04 07:21:17

Re: Solving Equations using the elimination method. Help!

#27 2012-07-04 07:24:46

Re: Solving Equations using the elimination method. Help!

#28 2012-07-04 07:29:14

Re: Solving Equations using the elimination method. Help!

#29 2012-07-04 07:35:53

Re: Solving Equations using the elimination method. Help!

#30 2012-07-04 07:45:16

Re: Solving Equations using the elimination method. Help!

#31 2012-07-04 07:48:54

Re: Solving Equations using the elimination method. Help!

#32 2012-07-04 07:51:24

Re: Solving Equations using the elimination method. Help!

#33 2012-07-04 07:56:49

Re: Solving Equations using the elimination method. Help!

#34 2012-07-04 07:58:03

Re: Solving Equations using the elimination method. Help!

#35 2012-07-04 08:00:58

Re: Solving Equations using the elimination method. Help!

#36 2012-07-04 08:07:19

Re: Solving Equations using the elimination method. Help!

#37 2012-07-04 08:13:26

Re: Solving Equations using the elimination method. Help!

#38 2012-07-04 08:15:37

Re: Solving Equations using the elimination method. Help!

#39 2012-07-04 08:16:29

Re: Solving Equations using the elimination method. Help!

#40 2012-07-04 08:31:07

Re: Solving Equations using the elimination method. Help!

#41 2012-07-04 08:34:13

Re: Solving Equations using the elimination method. Help!

#42 2012-07-04 08:36:51

Re: Solving Equations using the elimination method. Help!

#43 2012-07-04 08:38:09

Re: Solving Equations using the elimination method. Help!

#44 2012-07-04 08:42:08

Re: Solving Equations using the elimination method. Help!

#45 2012-07-04 08:44:52

Re: Solving Equations using the elimination method. Help!

#46 2012-07-04 08:49:57

Re: Solving Equations using the elimination method. Help!

#47 2012-07-04 08:57:45

Re: Solving Equations using the elimination method. Help!

#48 2012-07-04 09:07:07

Re: Solving Equations using the elimination method. Help!

#49 2012-07-04 09:29:30

Re: Solving Equations using the elimination method. Help!

#50 2012-07-04 10:31:59

Re: Solving Equations using the elimination method. Help!

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