This applet is designed to help students develop a visual intuition about the tangent line as a limit of secant lines.

• The applet has a control window and 2 graphics windows.
  
• The f(x) window shows a graph of the function in green. 
  
• There is a red dot on the graph and on the x-axis.  The dot on the axis is the hotspot and determines where we zoom in.  The hot spot can be dragged back and forth. 
  
• The yellow dot is at the red dot plus delta.  The value of delta can be controlled from the control panel.
• The blue secant line connects the red and yellow dot.  The slope of the secant line is displayed on the control panel.
  
• The magenta patch on the axis and graph is centered at the hot spot with a radius of delta.
• You have the option of displaying the tangent line in orange.
• The close up window zooms in on the hot spot plus or minus delta.  Notice how when delta is small the tangent and secant lines coincide.  (It is also worth noticing that when delta gets small enough, the graph of the function looks like a straight line that coincides with the tangent line.)