Vertex form of parabola; why x-h, not x+h?

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    Form Parabola Vertex
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Discussion Overview

The discussion centers around the vertex form of a parabola, specifically the expression y = a(x-h)² + k, and whether it could alternatively be expressed as y = a(x+h)² + k. Participants explore the implications of the minus sign in the vertex form and its significance in defining the vertex of the parabola.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants question the necessity of the minus sign in the vertex form, suggesting it may be a matter of tradition rather than mathematical necessity.
  • Others clarify that the vertex of y = a(x-h)² + k is (h, k), while the vertex of y = a(x+h)² + k would be (-h, k), indicating that a minus sign is required to correctly identify the vertex.
  • A participant draws a parallel between the vertex form of a parabola and the general equation of a line, questioning whether the subtraction in the vertex form is merely conventional.
  • Another participant notes that different forms of equations serve specific purposes, such as making certain characteristics of the function easier to identify, like the vertex in the case of parabolas.

Areas of Agreement / Disagreement

Participants express differing views on the necessity and implications of the minus sign in the vertex form. There is no consensus on whether the form could be altered to include addition instead of subtraction.

Contextual Notes

The discussion highlights the importance of conventions in mathematical expressions and the potential for confusion when altering standard forms. Participants do not resolve the implications of changing the sign in the vertex form.

Who May Find This Useful

Individuals interested in the properties of quadratic functions, students learning about different forms of equations, and those exploring mathematical conventions in algebra may find this discussion relevant.

Cicnar
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Hello.

The vertex form is y= a(x-h)^2+k, in general. Could it also be defined as y= a(x+h)^2+k?
I am wondering about that minus sign. I see no particular use of it. Is it there because of tradition
or am i missing something?
 
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The vertex of y= a(x-h)^2+k is (h,k)
The vertex of y= a(x+h)^2+k is (-h,k)

We need a minus somewhere.
 
Cicnar said:
Hello.

The vertex form is y= a(x-h)^2+k, in general. Could it also be defined as y= a(x+h)^2+k?
I am wondering about that minus sign. I see no particular use of it. Is it there because of tradition
or am i missing something?

When x=h, you have y=0+k=k
The vertex is h,k.
 
Thanks for your replies. But i think i was misunderstood. I will try to explain better this time.

For example, a general equation of a line is y=ax+b. What is special about addition operation? Is just a matter of convention? Could i say "a general equation of a line is given by y=ax-b"? I see nothing wrong with it.

Now, same logic for y= a(x-h)^2+k. This x-h part can be (or cant?) written as addition (x+h), if we choose to set our general equation in such form? Its a minor issue, but i was curios.
 
The different forms of equations make certain things easier to know about them.
y=mx+b, and y=ax^2+bx+c are the GENERAL form of a line, and of a parabola. They are easy to use for finding y values, and more convenient if using matrices. Ax+By=C, and y=a(x-h)^2+k are the STANDARD form for a line and for a parabola. The number-line intercepts are easy to identify for the line, and the vertex is easy to identify for the parabola, from the standard forms.
 
Oh, i see now! We can read more easily the desired information in this particular form (in this example, that is the coordinates of vertex). Makes perfect sense why this is the standard form now.

Thank you very much, symbolipoint.
 

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