How do I finish this polar equations problem?

[shreddinglicks]

Homework Statement

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

Homework Equations

r = 3cos(θ)

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)

 

[vela]

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