This demonstration illustrates the tangent line of a curve at a point. The points $a$ and $b$ can be dragged along the $x$-axis. The corresponding points $(a,f(a))$ and $(b,f(b))$ are joined by a line. As $b$ gets closer to $a$, this line approximates the tangent line of $f(x)$ at $x=a$. This process is animated by pressing the "b to a" button.