# Homework Help: Horizontal and Vertical Tangents for Polar Equation

1. Jan 16, 2014

### EnlightenedOne

1. The problem statement, all variables and given/known data
Find the points at which the following polar equation has horizontal and vertical tangents:

r2 = 4cos(2θ)

2. Relevant equations

$\frac{dy}{dx}$ = $\frac{r'(θ)sinθ + r(θ)cosθ}{r'(θ)cosθ - r(θ)sinθ}$

Horizontal Tangent: $\frac{dy}{dθ}$ = 0; $\frac{dx}{dθ}$ ≠ 0
Vertical Tangent: $\frac{dx}{dθ}$ = 0; $\frac{dy}{dθ}$ ≠ 0

3. The attempt at a solution
There is no "clean" way of solving for r (because of the +/- sqrt) so that I could find r'(θ) to use in the formula. So, I figured I would use implicit differentiation:

r2 = 4cos(2θ)

2r$\frac{dr}{dθ}$ = -8sin(2θ)

$\frac{dr}{dθ}$ = $\frac{-4sin(2θ)}{r}$

But, when I plug it in the formula, I now have an r and a θ when I set the numerator and denominator of $\frac{dy}{dx}$ equal to zero (separately of course). I don't know what to do when I have both variables like that and I'm trying to solve for θ. How should I be approaching this problem? Have I done this problem right so far? If so, what do I do next? If not, any suggestions? Please be clear. I can't find this problem answered clearly anywhere on the internet.
Thank you

2. Jan 16, 2014

### EnlightenedOne

For some reason the text isn't showing up right on my computer after posting. Let me know if its the same for you so I can edit and try to fix it.

3. Jan 16, 2014

### haruspex

You should have an equation involving r and θ (no derivatives around). Your original equation for the curve also has those two variables. Two equations, two unknowns.