is the gradient of T. VT is a vector quantity, with three components; it is the generalized
derivative we have been looking for. Equation 1.35 is the three-dimensional version of
Eq. 1.33.
Geometrical Interpretation of the Gradient: Like any vector, the gradient has magnitude
and direction, To determine its geometrical meaning, let's rewrite the dot product (l.35) in
abstract form: