This applet illustrates the concept of directional derivative. [br][br][list=1][*]Set the coordinates of point [math]P[/math] with the X and Y sliders.[br][/*][*]Set the direction of the unit vector [math]u[/math] with the Angle slider. [br][/*][*]Observe the curve that results from the intersection of the surface of the function with the vertical plane corresponding to [math]u[/math].[br][/*][*]The directional derivative of the function [math]f\left(x,y\right)[/math] at the point [math]P[/math] along the direction of the vector [math]u[/math] is the slope of the tangent line to the previous curve at [math]P[/math].[br][/*][/list][br]Change the function and repeat the previous steps.

Get more info about directional derivatives at [url=http://aprendeconalf.es/calculus/manual/derivatives-n-variables.html#Directionalderivative]http://aprendeconalf.es[/url].