# Parameterization, Folium of Descartes, etc.

1. Sep 11, 2012

### dr721

1. The problem statement, all variables and given/known data

Consider the surface z = f$\left(x y\right)$ = xy. Given a point P = $\left[a, b, ab\right]$ on this surface, show that the lines with direction vectors u = $\left[1, 0, b\right]$ and v = $\left[0, 1, a\right]$ through P are entirely contained in the surface.

2. The attempt at a solution

To be honest, I'm not really sure what I'm supposed to do here or how to go about doing it, if someone could explain it and/or give me a starting point, that'd be very helpful.

1. The problem statement, all variables and given/known data

Consider the curve given by x3 + y3 = 3axy.

What happens as t$\rightarrow$-1? Consider the limit as t$\rightarrow$-1 of x(t) + y(t). What do you conclude?

The curve has obvious symmetry. Verify this using your parametrization.

2. Relevant equations

The parametrized equations are:

x = 3at / 1 + t3

y = 3at2 / 1 + t3

2. The attempt at a solution

For the limit, after adding and simplifying the equations, it equals -a. I believe the equation is asymptotic at t = -1, but I guess I'm not quite sure what I'm supposed to conclude or what -a means.

For the symmetry part, it is symmetric about y = x, so that for every point (x, y) there is a point (y, x). How do I show this with the parametric equations?

2. Sep 12, 2012

### dr721

Sorry for the poor formatting. I don't know how to format column vectors or fractions.