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

johnD
06-15-2006, 09:45 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, 10:08 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, 10:39 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, 10:57 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
06-16-2006, 12:08 AM
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
06-16-2006, 12:12 AM
THANKS ALOT!!!!!!