This is probably a stupid question basic equation laws

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Discussion Overview

The discussion revolves around the manipulation of algebraic equations, specifically focusing on the process of isolating a variable (y) in the equation 9y - 4x + 5 = 0. Participants explore the implications of moving terms across the equation and the associated changes in signs and operations.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant asks whether the sign changes when moving the term 9 to the other side of the equation, indicating uncertainty about the rules of algebraic manipulation.
  • Another participant explains that moving 5 to the right side involves subtracting it, which changes its sign, but emphasizes that moving 9 involves dividing both sides by 9, which does not change its sign but alters its position from the numerator to the denominator.
  • A third participant expresses frustration with the terminology of "moving" numbers, arguing that it can lead to confusion regarding whether to add or subtract, and stresses the importance of understanding the operations involved.
  • A later reply supports the previous point, highlighting the need for clarity in mathematical language to prevent misunderstandings among students.

Areas of Agreement / Disagreement

Participants generally agree on the importance of precise language in mathematics and the operations involved in manipulating equations. However, there is no consensus on the terminology used to describe these processes, as some find it ambiguous and potentially misleading.

Contextual Notes

The discussion reflects varying interpretations of algebraic manipulation and the potential for confusion stemming from informal language. There are unresolved aspects regarding the best practices for teaching these concepts effectively.

dvmckay23
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Okay, so say I have an equation that looks like this:

9y - 4x + 5 = 0
and I wan to move every thing but the y over to the right hand side.

When it gets to the last step and looks like this:
9y = 4x - 5

When I move 9 over to the other side, does it's sign change? I know the other terms will be divided by it, but I don't remember if it is supposed to be negative... my text has examples that aren't quite clear.

Thanks!
 
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[tex]y = \frac {4x - 5} {9}[/tex]

To move 5 from the left to right you have substracted 5 from both sides - in effect it has changed the sign, but all you did was to make the same operation to both sides of the equation.

To move 9 to the right side you have to divide both sides by 9. Sign won't change, but 9 will move from nominator to denominator. In a way that's similar to sign change.
 
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This is why I strongly dislike talking about moving numbers from one side of the equation to the other! People get confused about changing sign because "moving" doesn't distinguish between add and dividing.

When you "move" the x in x+ y= 5 to the right, what you are really doing is subtracting x from both sides of the equation. You should be thinking, "the x in x+ y is added to y so I need to do the opposite, subtract it from both sides:
x+ y- x= 5- x which reduces to y= 5- x. x didn't "change sign", it is subtracted.

If you have xy= 5 and want to get y alone, because x is multiplied, again you have to do the opposite: divide. xy/x= 5/x which is just y= 5/x.
 
Halls, great answer. It is so important that teachers of algebra pay particular attention to what they say they are doing, and their language. They then need to impose the same strictness on their students. (Easier said then done, I know, but in a perfect world...). "Moving a number to the other side" is way to ambiguous for mathematics. This causes students to get confused; case and point the OP.
 

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