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Simultaneous equations

2771024

How would I solve this ?

2x + 3y = 19
-2x + y = 1

The signs are the same so do I do (1) - (2)

Reply 1

Potatoo

Original post by JackT2000

How would I solve this ?

2x + 3y = 19
-2x + y = 1

The signs are the same so do I do (1) - (2)

You have +2x in the first one and -2x in the second one. You'd add them to rid the x and then solve it for y.

Reply 2

LifeIsFine

2--2=4
To eliminate x you need to add the two equations.

Reply 3

2771024

Original post by Potatoo

You have +2x in the first one and -2x in the second one. You'd add them to rid the x and then solve it for y.

But if when the signs are the same you subtract and when they are different you add?

Reply 4

Potatoo

Original post by JackT2000

But if when the signs are the same you subtract and when they are different you add?

Yes that is right.

Reply 5

2771024

Original post by LifeIsFine

2--2=4
To eliminate x you need to add the two equations.

So do I add the first equation to the second equation?

Reply 6

RDKGames

Study Forum Helper

Original post by JackT2000

So do I add the first equation to the second equation?

Original post by JackT2000

But if when the signs are the same you subtract and when they are different you add?

You only consider this depending on which variable you are attempting to eliminate, and the quantity of the variable needs to be the same in both equations. The signs of x are the different and the quantity's the same so you add them together.

No point in remembering rules if you don't understand why something is happening.

Reply 7

Potatoo

It doesn't matter what equation you add you'll get the same answer:
2x + 3y = 19
-2x + y = 1 WHEN YOU ADD YOU WILL GET RID OF THE 2X AND THE - 2X. think of it being -2+2=0

Then you will be left with

4y = 20 SINCE WE ARE ADDING. NOW DIVIDE BOTH SIDES BY 4
y= 5 NOW TO GET x SUB IN Y=5 IN EITHER EQUATION AND YOU SHOULD GET x = 2
Hope this helps a bit better

Reply 8

2771024

Original post by RDKGames

You only consider this depending on which variable you are attempting to eliminate. The signs of x are the same so you subtract.

No point in remembering rules if you don't understand why something is happening.

So how would I show this on paper?

Reply 9

RDKGames

Study Forum Helper

Original post by JackT2000

So how would I show this on paper?

Add the two equations and show how the x variable disappears from this operation.

Reply 10

2771024

Original post by RDKGames

Add the two equations and show how the x variable disappears from this operation.

What I mean is how would I show this as I am adding the x variables together and taking away the y variables from each other

Reply 11

RDKGames

Study Forum Helper

Original post by JackT2000

What I mean is how would I show this as I am adding the x variables together and taking away the y variables from each other

Why are you subtracting the y variables?

Reply 12

2771024

Original post by RDKGames

Why are you subtracting the y variables?

Because the signs are the same for the y variables. You subtract if the signs are the same ?

Reply 13

RDKGames

Study Forum Helper

Original post by JackT2000

Because the signs are the same for the y variables. You subtract if the signs are the same ?

So what? You clearly don't understand the concept then.

The only reason why you're adding is to make the x's cancel, we don't consider the y's at this point, doesn't matter what their signs are. We are only interesting in the signs of the x variable since that's the one we are eliminating.