How does the elimination method work for solving simultaneous equations?

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Discussion Overview

The discussion centers around the elimination method for solving simultaneous equations, exploring its foundational principles and the reasoning behind adding or subtracting equations. Participants also touch upon the substitution method and seek clarity on the graphical interpretation of these methods.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Homework-related

Main Points Raised

  • Some participants explain that the elimination method relies on the principle that if two expressions are equal, performing the same operation on both sides maintains the equality.
  • One participant illustrates this with an example, showing how adding equations can eliminate a variable, leading to a solution for the remaining variable.
  • Another participant emphasizes the importance of understanding that the balance of equations is maintained regardless of the specific values, as long as the operations are consistent on both sides.
  • There is a discussion about the graphical interpretation of simultaneous equations, where setting two equations equal to each other helps find their intersection point.
  • Some participants express confusion about the transition from setting equations equal to solving for a variable, seeking step-by-step clarification.

Areas of Agreement / Disagreement

Participants generally agree on the foundational principles of the elimination method, but there is no consensus on the intuitive understanding of the steps involved in solving equations or the graphical representation of the solutions.

Contextual Notes

Some participants mention specific forms of equations and their implications, but the discussion does not resolve the nuances of these forms or their strict definitions.

Who May Find This Useful

This discussion may be useful for students learning about methods for solving simultaneous equations, educators looking for different explanations, and anyone interested in the conceptual underpinnings of algebraic methods.

roger
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I need a math guru to explain why and how the elimination method of solving simultaneous equations works ?

why do we add or subtract the two equations ?(I undertand in order to eradicate either term) but I need to know from the basics .

For that matter, how/why does the substitution method work ?

thanks

Roger
 
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roger said:
I need a math guru to explain why and how the elimination method of solving simultaneous equations works ?

why do we add or subtract the two equations ?(I undertand in order to eradicate either term) but I need to know from the basics .

For that matter, how/why does the substitution method work ?

thanks

Roger

The answer really goes back to the nature of "=". Think of it as weighing things on balance scale. If, when you put A on one side and B on the other, if they balance, then A= B (that is, A and B weigh the same). If you add the same weight on both sides, they still balance. If you take the same weight from both sides, they still balance.

If I know that x+ y= 10 then adding x+ y to one side of any equation is the same as adding "10". In particular, if I also know that x- y= 6, then
adding x+ y to x- y gives me 2x. Adding 10 to 6 gives me 16 and that's STILL the same thing: 2x= 16 so I must have x= 8.

Long, long ago, you learned "if you do the same thing to both sides of an equation, you still have a true equation". It's still true!
 
roger:
Note that, just as with the scales, a balance can be sustained even if the weight on a single side has changed (as long as the weight on the other side also has changed accordingly).
It is the condition that the weights on BOTH scales are EQUAL which keeps the balance; the particular value of that shared weight has no bearing on the balancing.
 
HallsofIvy said:
The answer really goes back to the nature of "=". Think of it as weighing things on balance scale. If, when you put A on one side and B on the other, if they balance, then A= B (that is, A and B weigh the same). If you add the same weight on both sides, they still balance. If you take the same weight from both sides, they still balance.

If I know that x+ y= 10 then adding x+ y to one side of any equation is the same as adding "10". In particular, if I also know that x- y= 6, then
adding x+ y to x- y gives me 2x. Adding 10 to 6 gives me 16 and that's STILL the same thing: 2x= 16 so I must have x= 8.

Long, long ago, you learned "if you do the same thing to both sides of an equation, you still have a true equation". It's still true!

What about the fact that the addition of the equation makes the y disappear, is that an optical illusion, or a false alarm ?
 
roger said:
What about the fact that the addition of the equation makes the y disappear, is that an optical illusion, or a false alarm ?

Nope,it's none of them.It's something natural.Take the system of equations:
[itex]x-y=7;y=3[/itex].Consider the first equation:[itex]x-y=7[/itex].Add on both sides "y".U'll get:[itex]x-y+y=7+y[/itex].Reduce "y" in the LHS and you're left with:[itex]x=7+y[/itex].Use the second equation to get:[itex]x=7+3=10[/itex].
Eliminating variables/unknowns is the purpose of adding/subtracting equations.As Halls said,it is based upon the mathematical significance of the sign "=".

Daniel.
 
The basics:
To solve equations simultaniously you need to set them equal to each other.

It helped me to think of it graphically.
When you set two equations of 2 lines (y=mx+c) equal to each other you will find the intersecting point (the solution).
 
why are the equations given in the form ay + bx = c ?

is the form given above strictly a function ?
 
What I also need an explanation for is :
in dexterciobys example :

y = x-7 and y = 3

Now I set y = x-7 = 3

he said add 7 to both sides , x=10 which is the value for which f(x) is the same .
or the point where they cross,

BUT from the step, where I set the y to equal both x-7 and 3 , to the step where I find out the value of x which gives the same values for f(x) is still not intuitive in my mind.
I need someone to show me step by step ?

thanx

roger
 
two things that are each equal to something else are equal to each other.
If x-7=y and 3=y, then x-7=3.
 
  • #10
Some people like to explain it this way to a class: Y=X-7 = 3
Here, we try to get under that line, and we draw a line +7=+7
Then by adding we get X=3+7 = 10.
 
Last edited:

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