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Directional Derivatives and Gradients (Three Variables)

Directional derivatives and gradients for functions of three variables are similar to the case of two variables.


Given a real function

w = f(x, y, z)

and a unit vector

U = cos αi + cos βj + cos γk

in space, the derivative of f in the direction U and the gradient of f at (a, b, c) are defined as follows.



Suppose w = f(x, y, z) is smooth at (a, b, c). Then for any unit vector

U = cos αi + cos βj + cos γk,

the directional derivative fU(a, b, c) exists and


Corollaries 1 and 2 also hold for functions of three variables. In Corollary 2, grad f is normal to the tangent plane of the level surface f(x, y, z) - f(a, b, c) = 0 at (a, b, c).

Last Update: 2006-11-25