## Quick Reference

(on a line)

One way of assigning coordinates to points on a line is as follows. Make the line into a directed line by choosing one direction as the positive direction, running say from *x*' to *x*. Take a point *O* on the line as origin and a point *A* on the line such that *OA* is equal to the unit length. If *P* is any point on the line and *OP*=*x*, then *x* is the coordinate of *P* in this coordinate system. (Here *OP* denotes the measure.) The coordinate system on the line is determined by the specified direction of the line, the origin and the given unit of length.

*Subjects:*
Mathematics.

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