Notice that all the secant lines in the figure pass through the given point {{MATH_104}}. What distinguishes these lines are their slopes; the slope of each line is determined by an additional point on the line, {{MATH_105}}. The figure suggests that``the limit of slopes of secant lines" is the slope of a special line, called the {{CMD_106}}tangent line{{CMD_107}} to the curve {{MATH_108}} at the point {{MATH_109}}. Notice how the tangent line and the curve are indistinguishable near the point {{MATH_110}}. If one can ``zoom in'' on the graph at {{MATH_111}} sufficiently so that it appears to be a straight line, then that line is the {{CMD_112}} to {{MATH_113}} at the point {{MATH_114}}. This is illustrated in the figure below.