5x-y=9

x-1.5y=-3

i dont even kno what it means by elimination. i know how to solve by substitution, but not elimination....

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- Jan 18th 2009, 08:55 PMannabananzaasolve by elimination
5x-y=9

x-1.5y=-3

i dont even kno what it means by elimination. i know how to solve by substitution, but not elimination.... - Jan 18th 2009, 10:22 PMGrandadSubstitution
- Jan 18th 2009, 10:35 PMannabananzaa
so i take the first problem, turn it into y=5x-9 and then just plug that into the first equation so it looks like x-1.5(5x-9)=9

would that be right? - Jan 18th 2009, 10:43 PMGrandadSubstitution
- Jan 18th 2009, 10:53 PMannabananzaa
of course i will!

so this is what im coming up with step by step, and it doesnt seem right still....

x-1.5(5x-9)=9

x-7.5x+13.5=9

-6.5x+13.5=9

-6.5x=-4.5

-4.5/-6.5

x=.69? - Jan 18th 2009, 11:15 PMmr fantastic
Crazily enough, it's not ..... Elimination and substitution are taught as two seperate methods.

One way of applying the elimination method:

$\displaystyle 5x - y = 9$ .... (1)

$\displaystyle x - 1.5y = -3$ .... (2)

Multiply equation (2) by -5:

$\displaystyle -5x + 7.5y = 15$ .... (3)

Now add equation (1) and equation (3) to**eliminate**$\displaystyle x$:

$\displaystyle 6.5 y = 24 \Rightarrow y = \, ....$

Now substitute the value of $\displaystyle y$ into any equation and solve for $\displaystyle x$.

This is what is taught in schools (so don't shoot*me*....) - Jan 18th 2009, 11:24 PMannabananzaa
so i was right when saying my answer looked a little crazy?

im sorry, im still confused... i dont see where you got the 3 from in the begining and then i dont get why i would add the equations.... - Jan 18th 2009, 11:30 PMmr fantastic
What 3? I have labelled the equations (1), (2) and (3).

You add so that x gets eliminated.

I have (reasonably) assumed that you have*some*familiarity with the method ....

Read this: Systems of Linear Equations: Solving by Addition / Elimination - Jan 18th 2009, 11:53 PMannabananzaa
so would my first answer that i posted be correct or this new answer that i got after following the steps on the link??

x=3.69

iim tryingg really hard!

just not very good at math :( - Jan 19th 2009, 12:25 AMmr fantastic
- Jan 19th 2009, 12:45 AMannabananzaa
ok. now i get it, i was forgetting the last step. thats what was really confusing me.

thank you so much for your patience!