$(15)$ Let $w=xyxzyz,$ where the variables $x,y,z$ are sot isdependent, bet constrained by a relation $y=f(x,z).$ Express $(\frac{\text{d}\text{e}\text{l}\text{w}}{\text{d}\text{e}\text{l}\text{y}}),$ in terms of $\stackrel{}{x},y,z$ and the formal partial derivatives $f,$ and $f.$ You can use either method: the chain'rule or differentials.

b$)$ gradv$=2i+3j$ at $(1,1)$; evaluate $\frac{ve}{\text{d}\text{e}\text{l}\text{r}}$ and $\frac{vd}{\text{d}\text{e}\text{l}}$ at this point.

$6.(15)$ Let $w=xy+xz+yz,$ where the variables $x,y,z$ are sot independent, but constrained by a relation $y=f(x,z).$ Express $(\frac{\text{d}\text{e}\text{l}\text{w}}{\text{d}\text{e}\text{l}\text{y}}),$ in terms of $x,y,z$ and the formal partial derivatives $f,$ and $f,.$

$(15)$ Let $w=xy+xz+yz,$ where the variables $x,y,z$ are not independent, but constrained by a relation $y=f(x,z).$ Express $(\frac{\text{d}\text{e}\text{l}\text{w}}{\text{d}\text{e}\text{l}\text{y}}{)}_x$ in terms of $x,y,z$ and the formal partial derivatives $f_x$ and $f_x.$ You can use either method: the chain rule or differentials.