having trouble understanding substitution this is the problem :

Solve for the system of equations by substitution

5x-7=-y

2x-y=0

This is another one..

6x+7y=1

x=55-9y

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- August 10th 2010, 08:49 PMmafai44solving for substitution
having trouble understanding substitution this is the problem :

Solve for the system of equations by substitution

5x-7=-y

2x-y=0

This is another one..

6x+7y=1

x=55-9y - August 10th 2010, 08:52 PMProve It

.

Substituting into the second equation gives

.

You know so

.

Therefore is the solution. - August 10th 2010, 08:54 PMeumyang
5x-7=-y

2x-y=0

First, solve one of the equations for one of the variables. It really doesn't matter which equation or which variable. You probably will want to choose the way that's the easiest. I'm going to start by solving the 2nd equation for y:

Now, substitute this into the 1st equation wherever you see a y, and then solve for x:

Now, plug this into the altered version of the 2nd equation:

The answer is (1, 2). You want to try your other example now?

EDIT: Too slow! ;) - August 10th 2010, 08:59 PMmafai44
thank you! does anyone know solving for elimination?

4x=15+3y

-6/5x+y=-17/5 - August 10th 2010, 09:01 PMProve It
I can't really read that...

Is it

? - August 10th 2010, 09:04 PMmafai44
yes the 6/5x , the x is for both the 6 and 5 not just the 5.. not even sure if that matters but you have it right.

- August 10th 2010, 09:12 PMProve It
Obviously it's not right if it's meant to be

since , not ...

Anyway...

Multiply the second equation by ...

.

Now add the equations together

.

Substituting into the first equation

.

So is the solution. - August 10th 2010, 09:19 PMmafai44
- August 10th 2010, 09:20 PMmafai44
double posted by accident sorry

- August 11th 2010, 04:12 AMeumyang
OP: Prove It's point is that you have to be careful with notation. Ideally, you should learn LaTeX so that it's not ambiguous when you type . 6/5x is really read as . If you are not using LaTeX and you want to indicate the fraction 6/5 times x, use parentheses:

**(6/5)x**.