Can we have parametric equations involving trigonometric functions for parabola

analytic geometryconic sectionsparametric

Usually the parmetric equations of a parabola, say $y^{2}=4ax$ are taken to be $y=2at$ and $x=at^2$. These do not involve any trigonometric functions, why? when all other conic sections (ellipse, circle, hyperbola) have parametric equations that involve trigonometric functions. Is it possible to have parameters for parabola too that inolve trig? If no then why not?

Also, is this a stupid question?

Best Answer

Not a stupid question at all.

I'd say that the hyperbola is usually parameterized with hyperbolic sines and cosines, so maybe not exactly "trig", but let's ignore that for a moment.

One thing you can do is say that there's a linear transformation from a circle to an ellipse (e.g., scale along one axis or the other or both, or even rotate a bit). In the simplest case, this converts the $(\cos t, \sin t)$ parameterization for the circle $x^2 + y^2 = 1$ into a $(\cos t, 2\sin t)$ parameterization for the ellipse $x^2 + (y/2)^2 = 1$.

If we could find a nice transformation from, say, a circle to a parabola, we could pull off the same trick. (It's actually more helpful to have a transformation from the parabola to the circle: you notice in my example how there were both a factor of $2$ and a factor of $1/2$? It'd be nicer if those were both $1/2$s, or both $2$s, and having the transform in the other direction makes that work.)

Let's look at the projective transformation $(x, y) \mapsto (\frac{x}{y-1}, \frac{y}{y-1})$. (You may well be observing that this isn't even defined on the line $y = 1$, and that's right --- projective transformations often have a domain that's 'missing a line' when written in standard $\mathbb R^2$ coordinates, and there's a whole complicated story there that you can learn about while looking at a projective geometry book; I love Hartshorn's "introduction to projective geometry" (or some name like that)).

Suppose we "substitute" that transformation into the equation of a circle, i.e., replace each $x$ with $x/(y-1)$, and similarly for $y$. We get $$ x^2 + y^2 = 1 \to \\ \frac{x^2}{(y-1)^2} + \frac{y^2}{(y-1)^2} = 1 \\ x^2 + y^2 = (y-1)^2 \\ x^2 + y^2 = y^2 - 2y + 1 \\ x^2 + 2y - 1 = 0 $$ which is the equation of a parabola.

Now if we knew an "inverse" to that projective transformation we'd be in good shape. Fortunately (for this contrived example), the inverse is exactly the same transformation! That means that since $(\cos t, \sin t)$ is a point of the circle, the point $$ (\frac{\cos t}{\sin t -1 }, \frac{\sin t}{\sin t -1 }) $$ is a point of our parabola. And there is a parameterization of the parabola by sines and cosines. You might object that when $\sin t$ is 1, the quotients are undefined, etc. That just means that the domain of the parameterization needs to be, say $-3\pi/2 < t < \pi/2$.

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