Log in

View Full Version : How do I solve using eliminations?


Kiwiwalter
Jul 8, 2010, 06:49 AM
3x+4y=-1, -9x-4y=-1

Curlyben
Jul 8, 2010, 06:52 AM
Nice and simple really.
Make x the subject in one of the equations then plug that value into the second.

MathMadMan
Jul 8, 2010, 07:03 AM
Homework?

First, tell us what the eliminations method is, then you'll see that the answer is right there.

Kiwiwalter
Jul 8, 2010, 07:14 AM
Well I don't really know because this is the first time I've ever seen this problem...

MathMadMan
Jul 8, 2010, 07:31 AM
Ok, by example:

4x + 3y = -5
2x - y = 5

Using the elimination method you multiple the second equation (both sides of it) by 3. This will make the -y in the second become -3y, so that when you add the two equations together, the y terms will cancel out.

Multiplying both sides of second equation by 3...
3(2x - y) = 3 x 5 => 6x - 3y = 15

This results in this pair of equations

4x + 3y = -5 <= the original first equation
6x - 3y = 15 <= new second equation

Adding them together (3y + -3y = 0)...

10x = 10

And thus x = 1

To solve for y, plug x=1 into the first equation (you could choose the second equation, just pick what looks easier)

4 + 3y = -5
3y = -9
y=3


So, in the elimination method, your goal is to get rid of one of the variables by multiplying or dividing one of the equations by a constant value and then adding the two equations together. If the equations already have equal but opposite terms, just add them together and solve for the remaining variable.