Let $f ( x , y , z ) = x ^ { 2 } + y ^ { 2 } + x z$

Short Answer

Let $f ( x , y , z ) = x ^ { 2 } + y ^ { 2 } + x z$ . Find the directional derivative of $f$ at $( 1,2,0 )$ in the direction of the vector $\mathbf { v } = \{ 1 , - 1,1 \}$ .