Polar coordinates: set the center of the graph as point 0, called the polar point, and draw a ray 0X from the polar point, called the polar axis, and then select a unit of length and the positive direction of the angle (usually take the counterclockwise direction). Take a point M on the graph, use ρ to denote the length of the line 0M, θ to denote the angle from 0X to 0M, ρ is called the polar diameter of the point M, θ is called the polar angle of the point M, the ordered number pair (ρ,θ)* is called the polar coordinates of the point M. The coordinate system established in this way is called the polar coordinate system.
