What is the focus and parameter of a parabola with vertex off the origin?

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  • #1
rudransh verma
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Problem in general eqn of parabola
The general eqn of parabola is ##(x-h)^2=-4a(y-k)##. This is the parabola whose vertex doesn't lie on origin and axis is parallel to y axis. It opens downwards. Vertex is (h,k). What will be the focus of this parabola and what is ##a## in general form?
In the diagram a<0 which is confusing.https://byjus.com/maths/standard-equations-of-parabola/
 

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  • #2
rudransh verma said:
Summary:: Problem in general eqn of parabola

In the diagram a<0 which is confusing.https://byjus.com/maths/standard-equations-of-parabola/
No, it is wrong. Parabolas of the form ##y=ax^2 +\ldots## are open at the top if ##a>0## and open at the bottom if ##a<0.## You can see this by yourself if you plug in some values for the parameters and the variable ##x.##

It could, however, also be the case that you put in a minus sign where there is none and the actual equation would be ##(x-h)^2=4a(y-k).##

You can find a lot of information on the Wikipedia page:
https://en.wikipedia.org/wiki/Parabola
 
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  • #3
fresh_42 said:
No, it is wrong. Parabolas of the form ##y=ax^2 +\ldots## are open at the top if ##a>0## and open at the bottom if ##a<0.## You can see this by yourself if you plug in some values for the parameters and the variable ##x.##

It could, however, also be the case that you put in a minus sign where there is none and the actual equation would be ##(x-h)^2=4a(y-k).##

You can find a lot of information on the Wikipedia page:
https://en.wikipedia.org/wiki/Parabola
If I open this up I get ##\frac{-x^2}{4a}+\frac{hx}{2a}+\frac{4ak-h^2}{4a}=y##
Where is a>0?
 
  • #4
rudransh verma said:
If I open this up I get ##\frac{-x^2}{4a}+\frac{xh}{2a}+\frac{4ak-h^2}{4a}=y##
What is this? If ##a<0## the coefficient at ##x^2## is positive, i.e. open at the top.
 
  • #5
rudransh verma said:
Summary:: Problem in general eqn of parabola

The general eqn of parabola is ##(x-h)^2=-4a(y-k)##. This is the parabola whose vertex doesn't lie on origin and axis is parallel to y axis. It opens downwards. Vertex is (h,k). What will be the focus of this parabola and what is ##a## in general form?
In the diagram a<0 which is confusing.https://byjus.com/maths/standard-equations-of-parabola/
The equation above is not the "general equation of a parabola." The page you linked to shows 8 different forms of the equation, depending on whether the vertex is at the origin (the first four) or where the vertex has been translated away from the origin.

The four equations in which the parabola opens to the left or right don't represent functions, so you're probably better off focusing on the ones that open upward or downward.
 
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  • #6
rudransh verma said:
whose vertex doesn't lie on origin
Aside from all other considerations about your post, you need to be aware that this is a science forum and precision of statements is important. That statement is NOT a generally true statement because it is false if h and k are zero. The general statement would be "whose vertex is at (h,k)".
 
  • #7
Mark44 said:
The equation above is not the "general equation of a parabola."
Then what is the general eqn of parabola? Is it ##y=ax^2+bx+c##
 
  • #8
rudransh verma said:
Then what is the general eqn of parabola? Is it ##y=ax^2+bx+c##
This is the general equation of a parabola that opens upward (a > 0) or downward (a < 0). It is not the equation of a parabola that opens either to the left or the right, so this equation is not the general equation of a parabola.

The equation you wrote is general (in the sense of upward-opening or downward-opening parabolas), but is not helpful at all in determining the vertex of a parabola.
 
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  • #9
rudransh verma said:
Summary:: Problem in general eqn of parabola

The general eqn of parabola is ##(x-h)^2=-4a(y-k)##. This is the parabola whose vertex doesn't lie on origin and axis is parallel to y axis. It opens downwards. Vertex is (h,k). What will be the focus of this parabola and what is ##a## in general form?
In the diagram a<0 which is confusing.https://byjus.com/maths/standard-equations-of-parabola/
I disagree with the diagram. If ##y \lt k##, then ##(y-k) \lt 0##. So if ##a \lt 0## and ##y \lt k##, the right-hand side, ##-4a(y-k)##, is negative. But the left-hand side, ##(x-h)^2##, is always positive.
 
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  • #10
Mark44 said:
The page you linked to shows 8 different forms of the equation,
You mean 8 forms of the general eqn of parabola which is?
 
  • #11
rudransh verma said:
You mean 8 forms of the general eqn of parabola which is?
There is no "general equation of a parabola." Period. If the parabola opens upward or downward, its equation is very different from one that opens to the left or to the right.

In my earlier post I said that the page you linked to showed 8 different equations, each of which shows one type of parabola. Again, the first four equations show parabolas whose vertices are at the origin, and the next four equations show parabolas whose vertices are at a point (h, k).

Notice that the title of the page you linked to is "Standard Equations of Parabola (sic)".
 
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  • #12
Mark44 said:
There is no "general equation of a parabola." Period. If the parabola opens upward or downward, its equation is very different from one that opens to the left or to the right.
But i don't understand where point (a,0) or (-a,0) or (0,a)... lies in last four eqns on the graph.
 
  • #13
rudransh verma said:
But i don't understand where point (a,0) or (-a,0) or (0,a)... lies in last four eqns on the graph.
Your confusion clearly stems from two things. First you don't actually understand the relationship between the equations and the graphs and second you clearly have not actually done the work to graph some of the equations yourself. The first problem stems directly from the second problem.

You say you don't know where "a" is. That's because "a" is a variable, as you would have immediately realized had you graphed one of the equations. If you make "a" big vs little, you get a signification different looking graph (but all still parabolas, just some are "wide" and some are "narrow" depending on what value you give to "a").

As usual, you don't seem to be taking a "bottom's up" approach to learning but rather insist on jumping into the middle of things before actually understanding the basics. It's an extraordinarily poor learning technique and will continue to serve you poorly.
 
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  • #14
phinds said:
You say you don't know where "a" is. That's because "a" is a variable
##a## is a variable, yes. One might call it a "parameter" that describes the parabola. It is the distance between the "vertex" of the parabola and its "focus".

A parabola can be described based on a given line (the "directrix") and a given point (the "focus"). The point is called the "focus". The parabola is the set of points that are at the same distance from the directrix as they are from the focus.
https://en.wikipedia.org/wiki/Parabola said:
1648058923518.png
For the parabola described by the equation: y = x^2, one can guess that the focus is at ##(0,\frac{1}{4}##) and that the directrix is at ##y=-\frac{1}{4}##. We can test this with a few values from the parabola.

##(x,y) = (0,0)##: Distance from focus = ##\frac{1}{4}##, Distance from directrix = ##\frac{1}{4}##.

##(x,y) = (\frac{1}{2},\frac{1}{4}##): Distance from focus: ##\frac{1}{2}##, Distance from directrix: ##\frac{1}{2}##.

##(x,y) = (1,1)##: Distance from focus = ##\frac{5}{4}##, Distance from directrix = ##\frac{5}{4}##.

##(x,y) = (2,4)##: Distance from focus = ##\frac{17}{4}##, Distance from directrix = ##\frac{17}{4}##.

Interesting way to generate Pythagorean triples.
 
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  • #15
jbriggs444 said:
la. It is the distance between the "vertex" of the parabola and its "focus".
Ok! But if it’s the distance how come it’s negative or positive?
phinds said:
If you make "a" big vs little, you get a signification different looking graph (but all still parabolas, just some are "wide" and some are "narrow" depending on what value you give to "a").
I have checked the graphs already. But the confusion is this that I mentioned above.
 
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  • #16
rudransh verma said:
Ok! But if it’s the distance how come it’s negative or positive?

I have checked the graphs already. But the confusion is this that I mentioned above.
Hi, @rudransh verma, I'm not native, could you explain me the first sentence of the quote?
Love
 
  • #17
Parabola.jpg
 
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  • #18
Lnewqban said:
Nice. Too bad that the OP is either unable or unwilling to do that for himself, as I had suggested to him.
 
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  • #19
The root problem is with the diagram shown in the OP, which contradicts itself. It says that ##a## is negative but then has a graph that does not match.
 
  • #20
FactChecker said:
The root problem is with the diagram shown in the OP
No, I think the root problem is that the OP has not tried to graph parabolas himself, using various versions of the equation/formula, to gain an understanding of the physical meaning of the various elements in the equations.
 
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  • #21
phinds said:
No, I think the root problem is that the OP has not tried to graph parabolas himself,
The equation given, with the statement in the diagram that ##a## is negative, does not match the graph. That is a problem.
 
  • #22
Discussion looks too wild and I cannot follow it too closely. At least someone discussed the use of the distance formula definition for a parabola. That coordinated from the proper picture/sketch should be extremely helpful. The rest is to continue study, exercise, and think.
 
  • #23
symbolipoint said:
Discussion looks too wild and I cannot follow it too closely. At least someone discussed the use of the distance formula definition for a parabola. That coordinated from the proper picture/sketch should be extremely helpful. The rest is to continue study, exercise, and think.
This discussion has wandered from the original OP question. The original question is about the bottom diagram in the linked article. That diagram is wrong. A negative ##a## in the equation shown can not give the graph shown.
 
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  • #24
FactChecker said:
This discussion has wandered from the original OP question. The original question is about the bottom diagram in the linked article. That diagram is wrong. A negative ##a## in the equation shown can not give the graph shown.
Yes! I think it’s wrong too. But how come a website like this has mistakes. It’s quite popular in India now a days.
It should open up.
 

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  • #25
rudransh verma said:
Yes! I think it’s wrong too. But how come a website like this has mistakes. It’s quite popular in India now a days.
It should open up.
Proofreading is a never-ending, thankless job. No matter how long and hard they work, some errors will remain. You did well to spot the error.
 
  • #26
FactChecker said:
Proofreading is a never-ending, thankless job. No matter how long and hard they work, some errors will remain. You did well to spot the error.
Ok. Thank you.
 
  • #27
rudransh verma said:
But how come a website like this has mistakes. It’s quite popular in India now a days.
As I said: Maybe the equation actually was ##(x-h)^2=+4a(y-k).## (The link does not open here.)
 
  • #28
fresh_42 said:
As I said: Maybe the equation actually was ##(x-h)^2=+4a(y-k).## (The link does not open here.)
Yes, that would fix the figure in the linked article. Probably better would be if it said ##a \gt 0## to conform to the interpretation of ##a## as a distance. But the figure is wrong as it stands. Some other figures that say ##a \lt 0## are wrong.
(The link works in my Firefox browser)
 
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  • #29
FactChecker said:
(The link works in my Firefox browser)
You don't have the BCP handicap (Brussel's chair pooper = EU).
 
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  • #31
FactChecker said:
Proofreading is a never-ending, thankless job. No matter how long and hard they work, some errors will remain. You did well to spot the error.
Mistakes happen, and a few of them are published. A student on some rare occasion will ask for help somewhere, tell his audience what "the book" or "the answer key" says, and upon some helper checking on the problem, finds that the book or key answer is wrong. Yes! Mistakes in answer key publishing happen!
 
  • #32
fresh_42 said:
No, it is wrong. Parabolas of the form ##y=ax^2 +\ldots## are open at the top if ##a>0## and open at the bottom if ##a<0.## You can see this by yourself if you plug in some values for the parameters and the variable ##x.##

It could, however, also be the case that you put in a minus sign where there is none and the actual equation would be ##(x-h)^2=4a(y-k).##

You can find a lot of information on the Wikipedia page:
https://en.wikipedia.org/wiki/Parabola
Yes - if you include a minus sign in the "general" equation then when a is positive the graph opens downward which is incorrect. This is like textbooks which insist on writing the equation of a parabola in turning point form as $$y=a(x+p)^2+q$$ instead of $$y=a(x-p)^2+q$$. We should be using the latter so that a positive value of p results in a right shift.
 
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  • #33
neilparker62 said:
Yes - if you include a minus sign in the "general" equation then when a is positive the graph opens downward which is incorrect. This is like textbooks which insist on writing the equation of a parabola in turning point form as $$y=a(x+p)^2+q$$ instead of $$y=a(x-p)^2+q$$. We should be using the latter so that a positive value of p results in a right shift.
If that "a" is positive value, then the parabola has a minimum point. If "a" is negative, then parabola has a maximum point.
 
  • #34
symbolipoint said:
If that "a" is positive value, then the parabola has a minimum point. If "a" is negative, then parabola has a maximum point.
Yes - that's correct and that's how it should be. eg ##y=x^2## with a=1 !
 

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