# Homework Help: Simultanous Equation

1. Nov 26, 2013

### smalls

so i got this question;

-4P1 + 6P2 =54
5P1 + 4P2=48

and my answer has to be to 3 decimal places.i got abit cofused at a point.but i have worked it out though..it got P1= -1.565 and P2= -13.956...

just need some one who's really good at this to double check it for me..if its wrong,then please let me know ....sorry for adding my workout..too long to do so

2. Nov 26, 2013

### Staff: Mentor

I moved your thread from the technical math section to here. Homework-type problems must be posted in the Homework & Coursework section.
Why can't you check it for yourself? Just substitute your values of P1 and P2 into your system. If your solutions are correct, you should get two true statements.

Being as you round your solutions, when you substitute your values, the left sides won't be exactly equal to the right sides, but they shouldn't be too far off.

3. Nov 26, 2013

### Staff: Mentor

Copied from the other thread you started.

4. Nov 26, 2013

### Staff: Mentor

What does this mean?

5. Nov 26, 2013

### Staff: Mentor

To eliminate the 2nd equation add 5 times the first equation to 4 times the second equation.

6. Nov 26, 2013

### SteamKing

Staff Emeritus

Your solutions are obviously wrong. You have P1 and P2 both as negative numbers. By inspecting the second equation with your purported solutions, they cannot possibly be correct since 48 is positive, and adding two negative numbers can only result in another negative number.

If you provide your calculations, we might be able to figure out where you went wrong.

7. Nov 26, 2013

### Staff: Mentor

Yes. What do you do next?

8. Nov 27, 2013

Look at this for example,

$2x-3y=20$
$3x+4y=40$

What do you do?
$3(2x-3y)=3(20)$
$2(3x+4y)=2(40)$

Which gives:
$6x-9y=60$
$6x+8y=80$
We can now cancel out 6 by multiplying the whole 2nd equation with -1
Which gives: $-6x-8y=-80$

Then add the two equations together:
$-17y=-20$
$y=\frac{20}{17}$

We have got y, Substitute y in one of the equations and find x:Which gives $x=\frac{200}{17}$

When we substitute x and y in both equations,it should turn out to be true

That's how we usually solve simultaneous equations