
2.3.15 Gradient and directional derivative
Problem:
Function \( z=z(x, y) \), point \( A\left(x_{0} ; y_{0}\right) \) and vector \( \vec{a} \) are given.
It is required to find at point \( A \) :
a) the gradient of the function and its value,
b) the derivative of \( z \) in the direction of vector \( \vec{a} \).
\[
z(x, y)=x^{2}-x y+y^{2}, \quad A(2 ; 2), \quad \vec{a}=\{6 ; 8\} .
\]