Rou can be positive or negative, but for convenience, rou is taken as the length of this line segment, which is generally positive.

θ is the polar angle, which is defined as the counterclockwise rotation angle from the positive direction of the X axis in the polar coordinate system. For example, if the X axis is rotated 90 degrees counterclockwise to get the positive direction of the Y axis, then the polar angle rou of the point in the positive direction of the Y axis is π/2. In the second picture, you can take a ruler, one end of which leans against the O point, and start to rotate from the positive direction of the X axis, counterclockwise is positive, clockwise is negative, until the ruler and the circle do not intersect, so,