# How do I finish this polar equations problem?

1. Sep 6, 2014

### shreddinglicks

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

Find the points on the given curve where the tangent is horizontal or vertical

2. Relevant equations

r = 3cos(θ)

3. The attempt at a solution

d/dθ = -3sin(θ)

for horizontal:

-3sin(θ)sin(θ) + 3cos(θ)cos(θ)

I used identity and got:

3cos(2θ) = 0

I got the values PI/4, 3PI/4

How do I get the values 3/sqrt(2) and -3/sqrt(2)

to give me points (3/sqrt(2), Pi/4) and (-3/sqrt(2), Pi/4)

2. Sep 7, 2014

### vela

Staff Emeritus
You found some values for $\theta$. Use the equation you started with to get the corresponding values for $r$.