Message-ID: <CANVKczM7znG6ZW4ksR_PByCRAGf0OJ3chOAq-KZwmoBc1Yuohg@mail.gmail.com>
Date: 2011-11-12T17:11:56Z
From: Barry Rowlingson
Subject: plotting a bivariate quadratic curve
In-Reply-To: <CAHfy0tWmMwcycp=zF1m3ascZVdTaoMt+JojcZnjWRCqx2qVVdg@mail.gmail.com>
On Sat, Nov 12, 2011 at 10:50 AM, Yackov Lubarsky <ylubarsk at gmail.com> wrote:
> Hi,
>
> I would like to plot a x,y curve described by the equation :
>
> Ax^2 + Bx + Cy^2 + Dy + E == 0
>
> (A,B,C,D,E are constants)
>
> This sounds like quite the common task but haven't been able to figure
> out how to do this. Could you please help ? I am new to R so probably
> missing something basic.
What you may be missing was all figured out by the ancient Greeks and
they didnt even have R in their alphabet!
You've got this:
http://mathworld.wolfram.com/QuadraticCurve.html
but with b=0. I'm not sure how many of the awkward cases (line pairs,
imaginary everything) disappear with b=0, but you can compute the
Delta, I, J, and K determinants to figure it out. Then set theta to
seq(0,2*pi,len=100) and compute r from the polar equation form. Plot
r*cos(theta), r*sin(theta)...
Confession: its been a long time since I did maths like this (forgive
me father, for I have not sin'd).
Barry