Directional Derivatives for the TI89     by: Jake Griffin (Jrock7286@aol.com)

This function returns the directional derivative of any multivariable function
in the direction of some vector u.  The syntax is as follows:

dderiv(function, variable list, vector u)

Examples:

Directional derivatives of x^2+y^2 in x then y directions:

dderiv(x^2+y^2,{x,y},[1,0]) ---> returns 2x which is the same as d(x^2+y^2,x)
dderiv(x^2+y^2,{x,y},[0,1]) ---> returns 2y which is the same as d(x^2+y^2,y)


In the direction of the line y=x:

dderiv(x^2+y^2,{x,y},[1,1]) ---> returns sqrt(2)*x + sqrt(2)*y

Note that the vector 'u' doesn't have to be a unit vector, and the function
does NOT have to be limited to x and y

For comments, questions, death threats, email me at Jrock7286@aol.com and I will
get back to you as soon as I can.  Enjoy!