Ok there's something simple I don't get here. When one double integrates a circle in polar coordinates or even cylindrical coordinates, sometimes one subs in x=rcostheta and y=rsintheta but sometimes x=3costheta and y=3sintheta even when we know both times that the radius of he circle is 3. Can anyone tell me how one knows when to plug in the radius and when just to leave it as r? I just got a question wrong because I put r instead of 3...