# Family of parabolas through three points?

I saw this question about a week ago. We were asked to derive the equation for the parabola through these three points in the Cartesian plane:

(1, 11), (0, 6), (2, 18)

My problem with the question is that there is more than one parabola fitting those three points. In fact, there are infinitely many. Everyone else was assuming a vertical axis, which is probably what the asker intended, but that condition was not stated. So what about the others?

Using a single variable parameter, derive an equation representing the family of parabolas passing through the three given points.

Please read it carefully. The objective is not a single parabola, but rather a family of parabolas. I asked this same question two days ago, but was compelled to delete it because nobody was addressing the question.

### 1 Answer

- RaffaeleLv 79 years ago
You need 5 points, in general, to determinate one and only one parabola

These 5 points cannot be anywhere in the plane

In the following drawing

http://img21.imageshack.us/img21/2898/0parabola.pn...

A,B and C are the given points (1, 11), (0, 6), (2, 18)

A generic conic section in a xy-plane has equation

ax^2 + bxy + cy^2 + dx + fy + g = 0 (*)

(dividing by one of the parameters which is not zero, actually they reduce to five)

1st condition is that (*) is not the equation of a degenerate conic section

like x^2 - y^2 = 0 which represents 2 straight lines

it happens when the plane passes through cone simmetry axis

(*) is not degenerate iff a determinant formed with its coefficients is not zero

look here for details

http://en.wikipedia.org/wiki/Matrix_representation...

2nd condition is that discriminant of (*) is zero

b^2 - 4ac = 0

as you can see it's very hard, with 3 points only, found the double parameters family of parabolas

If we knew that one of them is the vertex, it should be easier and the parabolas will deend by only one parameter

anyway

thank you for the interesting question

many teachers 'forget' to specify what is the symmetry axis

leaving it as an 'implicit' condition

Source(s): a great free software for any platform http://www.geogebra.org/cms/