Quadratic Formula: Positive or Negative Root?

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Discussion Overview

The discussion revolves around the interpretation of the quadratic formula, specifically the treatment of the "\displaystyle{\pm}" symbol in the context of finding roots of quadratic equations. Participants explore whether the positive root should be added to -b or subtracted from it, and clarify the existence of two distinct roots derived from the formula.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants express confusion about how to treat the "\displaystyle{\pm}" in the quadratic formula, questioning whether it indicates addition or subtraction of the positive root.
  • Others clarify that the quadratic formula yields two roots, and both the addition and subtraction of the square root term should be considered.
  • A participant mentions that regardless of whether the discriminant is a perfect square, the formula still provides two solutions, one for addition and one for subtraction.
  • There is a discussion about the notation of the square root, with some asserting that \sqrt{x} denotes only the positive root, while others caution against common misconceptions regarding square roots.

Areas of Agreement / Disagreement

Participants generally agree that the quadratic formula results in two roots, one from addition and one from subtraction. However, there is some disagreement and confusion regarding the interpretation of the "\displaystyle{\pm}" symbol and the implications of perfect squares on the roots.

Contextual Notes

Some participants express uncertainty about the implications of perfect squares on the roots, and there is a lack of consensus on the interpretation of the square root notation in relation to the quadratic formula.

Holocene
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Regarding [tex]\displaystyle{\frac{-b \pm \sqrt{b^2 - 4ac}}{2a}}[/tex]

How should "[tex]\displaystyle{\pm}[/tex]" be treated?

I know a square root can be both possitive and negative, but does the quardratic formula dictate that a possitive root should be added to -b, or does it dictate that a positive root should be subtracted from -b?
 
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The quadradic forumal yields 2 roots. You do both.
 
Some people get confused because other times they see the plus/minus sign, they have to choose correctly, whilst in this case you do it both. Thats why you may also see the formula written as;

[tex]x_1= \displaystyle{\frac{-b + \sqrt{b^2 - 4ac}}{2a}}[/tex]

[tex]x_2 = \displaystyle{\frac{-b - \sqrt{b^2 - 4ac}}{2a}}[/tex]
 
Integral said:
The quadradic forumal yields 2 roots. You do both.

Sorry, I'm a little confused.

Say a particular solution to a quadratic equation is [tex]\displaystyle{\frac{2 \pm \sqrt{7}}{3}}[\tex]<br /> <br /> If 7 was a perfect sqaure, would the root get added to or subtrected from 2?[/tex]
 
Holocene said:
Sorry, I'm a little confused.

Say a particular solution to a quadratic equation is [tex]\frac{2 \pm \sqrt{7}}{3}[/tex]

If 7 was a perfect square, would the root get added to or subtracted from 2?

It doesn't matter if 7 is a perfect square or not (and it isn't). There are TWO roots to a quadratic equation, hence the quadratic formula has TWO solutions. One of the solutions is when you add the square root part, the other solution is when you subtract it. Look at my previous post.
 
Gib Z said:
It doesn't matter if 7 is a perfect square or not (and it isn't). There are TWO roots to a quadratic equation, hence the quadratic formula has TWO solutions. One of the solutions is when you add the square root part, the other solution is when you subtract it. Look at my previous post.

Okay I've got it. The 2 solutions are adding a possitive root, and subtracting a possitive root, correct? (Subtracting a negative root is = to adding it, adding a negative root is = to subtracting it).
 
Holocene said:
Okay I've got it. The 2 solutions are adding a possitive root, and subtracting a possitive root, correct? (Subtracting a negative root is = to adding it, adding a negative root is = to subtracting it).
[itex]\sqrt{x}[/itex] is used to denote the positive square root of x. Thus [itex]\sqrt{4}=2[/itex] is correct while [itex]\sqrt{4}=\pm2[/itex] is incorrect. Thinking that the latter is correct is too common a mistake - make sure you aren't making it.
 
morphism said:
[itex]\sqrt{x}[/itex] is used to denote the positive square root of x. Thus [itex]\sqrt{4}=2[/itex] is correct while [itex]\sqrt{4}=\pm2[/itex] is incorrect. Thinking that the latter is correct is too common a mistake - make sure you aren't making it.

Got it, thanks.
 

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