1. The problem statement, all variables and given/known data Find the radius and center of sphere ρ = 28 cos ϕ. 2. Relevant equations Relevant equations would be the spherical and rectangular coordinate equations. 3. The attempt at a solution I started off by multiplying both sides of the equation by ρ to get ρ^2 = 28 ρ cosϕ Then this would allow me to get to the equation of a sphere which is x^2 + y^2 + z^2 = 28z I don't really know where to go from here. Could someone point me in the right direction? I can't really seem to find how to get rid of the z. I know the a^2 under x,y,z will give me the radius if I just take a.