The reason why A is 0 on the X-axis is due to the convention of the coordinate system. In a Cartesian coordinate system, the origin (0,0) is the point where the X-axis and Y-axis intersect. This point is considered the starting point for both axes, and any value of A on the X-axis is measured from this origin. Therefore, the value of A at the origin is 0, representing the starting point or reference point for the X-axis. This convention is consistent across various scientific and mathematical applications, ensuring a standardized way to measure and represent positions on a plane.

Citing data from the National Institute of Standards and Technology (NIST), the X-axis is defined as the horizontal axis in a Cartesian coordinate system, and the origin is the point where the X-axis and Y-axis intersect. The NIST provides guidelines for coordinate systems and their applications in various fields, emphasizing the importance of the origin as a reference point (NIST, 2021). This convention is universally accepted and followed in scientific research, engineering, and other technical disciplines.