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The center of the circle lies in the origin of the plane. Two dotted circles are inscribed around the origin of the plane. A vector r sub 1 represents the radius of the first circle C sub 1. The radius r sub 1 lies between the positive x and the positive y-axis, which makes an angle phi with the positive x-axis. A tangent line of the circle represents the unit vector phi caret. Another vector r sub 2 represents the radius of the second circle C sub 2.