To find the tangent line to a curve at a point we first pick another point on the curve a little bit away from . Call this point .
Do this below by clicking "Another Point".
Then connect these two points by a secant line (click "Secant Line").
The slope of this secant line can be computed since we know two points on the line. (click "triangle").
Move the slider to 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 heads to .