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Coordinate systems used in this paper. (a) 3-dimensional Cartesian coordinate system; the distance between O and P is d cm, the angle between OP and OX is β. (b) 2-dimensional Cartesian coordinate system; the case that P is located on the XOY plane, and α is the angle from the positive x-axis to PO. (c) the polar coordinate system in which P is taken as the origin, so that the position of O can be determined based on the position of P.
