1. The problem statement, all variables and given/known data The point is (0, -8, 0) r≥0 0≤θ≤2∏ 0≤[itex]\varphi[/itex]≤∏ 2. Relevant equations 3. The attempt at a solution So here is what I've done so far: I know that r=8 because x and z are 0 I know that θ=∏/4 or 3∏/4, but which one? both of these satisfy the following equation (which is the only one I know for this) cos(θ)=y/x Also, I am confused about [itex]\varphi[/itex]. I know that: tan([itex]\varphi[/itex])=y/x, but y is negative, and x is zero. I know the value of arctan(∞) as well as arctan(-∞) but which one do I use? I am so confused. Thanks!