Interactive: Tangent Line at a Point

Definition: The tangent line to the curve $y = f(x)$ at the point $P(a, f(a))$ is the line through $P$ with slope

$$m = \mathop {\lim }\limits_{x \to a} \frac{{f\left( x \right) - f\left( a \right)}}{{x - a}}$$

provided that this limit exists.

Instructions: With the mouse, move the x-value toward a to see that the tangent line is the limiting position of the secant line shown dashed. You can also move the points on the function. Refresh browser to start over.