err.....math help?

[gamarada717]

Today we were working on systems of equations, and we had a sub teach us the lesson, so I'm kind of confused. I'll try to work this out the best I can.

I have two linear equations. I have to solve them (find their common point, if they have one) by using substitution. This means no graphs. ok, here goes

my two equations are
3x-2y=0
x+y=-5
now, what she wants us to do is get x or y by itself, like
x=-y-5
now i substitute
3(-y-5)-2y=0
after working that, i got y=-3
now, I have to go back to the equation and plug in y to find my x, and then i have my x,y point. Only thing is I don't remember which equation to go back to, because they all give me different answers.

I'm sure this all is simple and you guys in highschool are laughing, but hey, we all forget to write a note down sooner or later, right? Thanks in advance.

[Miscue]

That looks right. You can use either equation, because the x and y values are supposed to be the same for both. That's the point of solving a system of equations... a common x and y value if those are your only variables.

Solving systems with hundreds or more variables = fun. That's a lot of what goes on in scientific computing, really. Computing large matrices.

[Emily]

You're on the right track. You've just made a mistake in basic math with everyone does.

y=-3

x+(-3)=-5
x+3-3=-5+3
x=-2

3x-2(-3)=0
3x+6=0
3x+6-6=0-6
3x/3=-6/3
x=-2

-2+(-3)=-5
-2-3=-5

3(-2)-2(-3)=0
-6+6=0


It's alway best to use the easiest equation but always work out both to check your answer.

[Miscue]

I don't see the mistake... besides not having solved for x yet...

[Emily]

Re: err.....math help?

Originally posted by gamarada717
Only thing is I don't remember which equation to go back to, because they all give me different answers.

I believe that to mean that she worked both equations but got 2 differnt answers for x.

[Miscue]

Ah! Now I see what you mean.

[gamarada717]

I remember her saying something about one case where you could only use one of the equations. Maybe I jsut dreamed that.

[gamarada717]

Re: Re: err.....math help?

Originally posted by Emily



I believe that to mean that she worked both equations but got 2 differnt answers for x.

Hold on....she? SHE?

WTF?

I am no women.....seriosuly offended.

[Emily]

Re: Re: Re: err.....math help?

Originally posted by gamarada717


Hold on....she? SHE?

WTF?

I am no women.....seriosuly offended.

I'm sorry

[Emily]

Originally posted by gamarada717
I remember her saying something about one case where you could only use one of the equations. Maybe I jsut dreamed that.

the only case i can think of is if one of the equations are false. but in that case you can't solve a false equation.

[gamarada717]

Lol, thanks, it's always nice to know that AO can always help me.