View Full Version : Please Help w/ solving 3x + 5y = -9, 4x + 2y = -68

johnD

06-15-2006, 08:45 PM

3x+5y=-9

4x+2y=-68

What is the solution to this system of equations?

What is the graphical meaning of the soluton?

I need help getting started on this

johnD

06-15-2006, 09:08 PM

i used the addition method and i came up with 168 for y, and that's where I got stuck because I know I'm wrong

stapel

06-15-2006, 09:39 PM

i used the addition method and i came up with 168 for y, and that's where I got stuck because I know I'm wrong

How did you get that value? How do you "know" it is incorrect?

Please reply showing your steps. Thank you.

Eliz.

johnD

06-15-2006, 09:57 PM

3x+5y=-9

4x+2y=-68

1st step - 4(3x+5y)=4(-9)

-3(4x+2y)=-3(-68)

2nd step - 12x+20y=-36

-12x-6y=204

y=168

( I think I'm wrong because 168 is too high of a number because then I have to graph it after I find the answer of x and y)

Mrspi

06-15-2006, 11:08 PM

3x+5y=-9

4x+2y=-68

1st step - 4(3x+5y)=4(-9)

-3(4x+2y)=-3(-68)

2nd step - 12x+20y=-36

-12x-6y=204

y=168

( I think I'm wrong because 168 is too high of a number because then I have to graph it after I find the answer of x and y)

You did GREAT up to this point:

12x + 20y = -36

-12x - 6y = 204

Now, add the two equations together:

12x + 20y = -36

-12x - 6y = 204

--------------------

xxxxx 14y = 168

Divide both sides by 14:

y = 12

Now, if y = 12, and if

3x + 5y = -9, then

3x + 5(12) = -9

3x + 60 = -9

3x = -69

x = -23

The solution should be (-23, 12).

The "graphical" interpretation of this solution is that the graphs of the two lines represented by the original equations should intersect at (-23, 12). Do NOT falll into the trap of thinking that a solution is incorrect because it would be difficult to graph (though your value for y in fact was not correct). One of the disadvantages of using graphs to solve systems is that not all equations are "convenient" to graph!

I hope this helps you.

johnD

06-15-2006, 11:12 PM

THANKS ALOT!!!!!!

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