# Axis of a Parabola

1. Dec 4, 2006

### chaoseverlasting

If someone could tell me how to go about it, I'd really appreciate it. Just point me in the right direction please. Kinda stuck.

The vertex of a parabola is (-3,1) and it touches the x axis at the origin. Find the equation of the axis of the parabola.

2. Dec 4, 2006

Did you mean, the parabola intersects the x axis at the origin? Or am I missing something?

3. Dec 4, 2006

### chaoseverlasting

No, the x-axis is the tangent to the parabola at the origin. I dont know what to do with it.

4. Dec 4, 2006

I dont think that possible.

5. Dec 4, 2006

### chaoseverlasting

Couldnt it be a tilted parabola of the type $$y^2=4ax$$ so that its lower arm touches the x-axis at the origin?

6. Dec 4, 2006

yes it could. In that case, it would be opening to the right.

7. Dec 4, 2006

### chaoseverlasting

So how would you find its axis?

8. Dec 4, 2006

it would simply be y = 1.

9. Dec 4, 2006

### chaoseverlasting

Why would it be y=1?

10. Dec 4, 2006

### arildno

Since the axis has to go through the parabola's vertex, and must be parallell to the x-axis.

11. Dec 4, 2006

### HallsofIvy

Why would the axis of the parabola have to parallel to the x-axis? We are told that the x-axis is tangent to the parabola at (0,0). No line parallel to the axis of a parabola is ever tangent to it.

12. Dec 5, 2006

### arildno

THe x-axis is tangent to the parabola at the origin.

The symmetry axis of the parabola is parallell to the x-axis, because the the tangent to the vertex is parallell to the y-axis.
The symmetry axis of a parabola is perpendicular to the tangent at the vertex.

13. Dec 5, 2006

### chaoseverlasting

It doesnt say the tangent at the vertex is paralell to the y-axis. You dont know the equation so you cannot assume that. the parabola could be tiled so that the axis of the parabola makes some angle with the x-axis and one of the arms just touches the x-axis at the origin.

14. Dec 5, 2006

### HallsofIvy

Yes, that is given.

Where was that said?

15. Dec 5, 2006

### chaoseverlasting

Where do you begin on something like this?

16. Dec 5, 2006

### arildno

I'm sorry, I was wrong.

17. Dec 5, 2006

### chaoseverlasting

18. Dec 5, 2006

### Hubert

Well, if it is a parabola, the equation is of the form y = f(x) = ax^2+bx+c

Now, f(-3)=1 and

f(0)=0

Finally, f'(0)=0

Also, since f(x)= ax^2+bx+c and f(0)=0 c must also equal zero.

EDIT: I think I'm wrong again (as usual). I guess the equation of a parabola does not have to be of the form ax^2+bx+c=f(x)

Last edited: Dec 5, 2006
19. Dec 6, 2006

### chaoseverlasting

Could it be that the question is incomplete?

20. Dec 6, 2006

### kishtik

I think the equation should be y=a(x-h)^2+k
Since (-3,1) is the vertex h=-3 and k=1
we get y=a(x+3)^2+1
If x=0 and y=0
9a+1=0 a=-1/9 y=1-((x+3)^2)/9

21. Dec 7, 2006

### chaoseverlasting

Yes, that would be true if the axis were parallel to the x-axis, but it isnt so. The parabola is tilted.

22. Dec 7, 2006

### uart

accidental double post.

Last edited: Dec 7, 2006
23. Dec 7, 2006

### uart

You can construct a geometric solution by using the property of a parabola that the gradient (tan of angle) of the "vertex to point" chord is equal to one half of the gradient of the tangent at the point.

See if you can follow through on this and after a bit of basic geometry you should be able to get an equation (based on the tan of sum/difference formula) of something like :

$$\frac{\tan(\theta)}{1 + 2 \tan^2(\theta)} = \frac{1}{3}$$

Actually that is the solution for the angle whose tan is one half that of the required angle of rotation, but it's going to be pretty easy if you can get to this point.

Note that the above is a quadratic so there are actually two solutions. One solution has smaller rotation (measured from vertical) and a longer focal length whereas the second solution has greater rotation (axis is closer to the x-axis) and a shorter focal length.

Last edited: Dec 7, 2006
24. Dec 9, 2006

### chaoseverlasting

Sorry, you lost me. The slope of the chord from a point to the vertex is equal to half the slope of the tangent at that point?

25. Dec 9, 2006

### uart

Yeah that's correct for a parabola of the usual form (x^2=4ay). Say you denote the vertex point "O" and some general point on the parabola as point "P", then the gradient of the straight line joining "O" and "P" is one half of the gradient of the tangent at "P". This a well known property of a parabola that's very easily proven.

Take a look at the attachment. This diagram shows the parabola after its been rotated back to a vertical axis. Your goal is to determine the angle PRQ though which we have rotated it to make it vertical. This of course will be the original angle of the axis (measured from vertical) of the original given parabola.

In this diagram we know the following facts.
1. tan(PRQ) = 2 tan(POR)
2. PS = 3
3. OS = 1
4. OSP is a right angle.

I used the above information and some geometry to solve for tan(POR) and hence to find angle PRQ.

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Last edited: Dec 9, 2006