To find the tangent line to a curve at a point [math]P[/math] we first pick another point on the curve a little bit away from [math]P[/math]. Call this point [math]Q[/math].[br][br]Do this below by clicking "Another Point".[br][br]Then connect these two points by a secant line (click "Secant Line").[br][br]The slope of this secant line can be computed since we know two points on the line. (click "triangle").[br][br]Move the slider to [math]0[/math] and observe how the secant lines approaches a line that just touches the curve at a single point. This line is called the tangent line. (click "tangent line" to show the tangent line). In other words, the secant lines approach the tangent line as [math]h[/math] heads to [math]0[/math].