How do you find the slope of the tangent line to a function at a point Q when you only have that one point? This Demonstration shows that a secant line can be used to approximate the tangent line. The secant line PQ connects the point of tangency to another point P on the graph of the function. As the distance between the two points decreases, the secant line becomes closer to the tangent line.

Usually, the slope of a line through two points and is calculated using the formula . This method, however, requires that you know two points; in the tangent line problem, you know only one point. In this case, the slope of the tangent line can be approximated through the use of a limit, , where is the horizontal distance between the point of tangency and another point. This Demonstration lets you manipulate the value of and shows how this affects the slope of the secant line.