# Homework Help: Simultaneous equations

1. Jan 25, 2012

### maali5

1. The problem statement, all variables and given/known data

How do yo solve simultaneos equations?

2. Relevant equations

Am I right ?

3. The attempt at a solution

y = x2 – 5x + 5 question a

5x + 3y = 30. question b

Find location of points collide?

x2 + 2y -25=0

x= -2+ square root(2^2-(4x1x(-25))/2 x = 4.09901

or

x= -2 - square root(2^2-(4x1x(-25))/2 x=-6.09

subsitue y etc

2. Jan 25, 2012

### Staff: Mentor

Looks like you are on the right track...

3. Jan 25, 2012

### maali5

Cheerio

4. Jan 25, 2012

### eumyang

I would use substitution. You already have equation a solved for y. Plug it into equation b:
5x + 3y = 30
5x + 3(x2 – 5x + 5) = 30
... and so on.

5. Jan 25, 2012

### Staff: Mentor

Errrm, if eumyang is on the right track, then I'll eat my hat!

... and I'll post the video of it on youtube.

6. Jan 25, 2012

### SammyS

Staff Emeritus
You can't use the quadratic formula on x2 + 2y -25=0, because it has two different variables in it.

B.T.W: Yesterday, Mark44 pointed out to you that you should not use the lower case letter, x, to indicate multiplication.

7. Jan 26, 2012

### maali5

Hey ladies ;) + gentleman :)

Now you have "REALLY" lost me.

I went with eumyang- subsituations and this is what I have done.

NB: Allright I WILL USE . FOR MULTIPLY NOW

Ok, lets start

1)

y = x^2 – 5x + 5 question a

5x + 3y = 30. question b

Find location of points collide?

5x + 3(x^2 – 5x + 5) = 30

5x+ 15x^2 -15x+15=30

15x^2-10x+15=30

15x^2-10x -15 = 30

5(3x^2 -2x -3) = 30

use formula

x= 2 + sqrt{40} / 6

x= 1.3874

or

x=2 - sqrt{40} / 6

x= -0.72075

Is this the right directions?

8. Jan 26, 2012

### SammyS

Staff Emeritus
The coefficient of the x2 term is 3, not 15.
Once you get a solution for x, you should go back to an original equation to find y. To check your answer for the pair x & y, you should then plug those values into the other original equation.

9. Jan 27, 2012

### Staff: Mentor

CORRECTION: I intended to write

10. Jan 27, 2012

### Staff: Mentor

You must make the right hand side = 0 before using the quadratic formula.

Taking as an example: 15x^2 - 10x - 15 = 30

Make RHS=0:

15x^2 - 10x - 15 - 30 = 30 - 30

x = (10 ± sqrt(100 + 4•15•45)) / 30