Let *XOX*′ and *YOY′* be two fixed perpendicular lines in the plane of the paper. The line *OX* is called the axis of *X* and *OY* the axis of *Y*. *OX* and *OY* together are called the coordinate axes. The point *O* is called the origin of the coordinate axes. Let *P* be a point in this plane. Draw *PM* perpendicular to *XOX*′. The distance *OM* is called the *x*-coordinate or abscissa and the distance *MP* is called the *y*-coordinate or ordinate of the point *P*.

If *OM = x* and *MP* = *y* then (*x, y*) are called the coordinates of the point *P*. The coordinates of the origin *O* are (0, 0). The lines *XOX*′ and *YOY*′ divide the plane into ...

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