Respuesta :
Answer:
The directional derivative of f at A in the direction of [tex]\vec{u}[/tex] AD is 7.
Step-by-step explanation:
Step 1:
Directional of a function f in direction of the unit vector [tex]\vec{u}=(a,b)[/tex] is denoted by [tex]D\vec{u}f(x,y)[/tex],
[tex]D\vec{u}f(x,y)=f_{x}\left ( x ,y\right ).a+f_{y}(x,y).b[/tex].
Now the given points are
[tex]A(8,9),B(10,9),C(8,10) and D(11,13)[/tex],
Step 2:
The vectors are given as
AB = (10-8, 9-9),the direction is
[tex]\vec{u}_{AB} = \frac{AB}{\left \| AB \right \|}=(1,0)[/tex]
AC=(8-8,10-9), the direction is
[tex]\vec{u}_{AC} = \frac{AC}{\left \| AC \right \|}=(0,1)[/tex]
AC=(11-8,13-9), the direction is
[tex]\vec{u}_{AD} = \frac{AD}{\left \| AD \right \|}=\left (\frac{3}{5},\frac{4}{5} \right )[/tex]
Step 3:
The given directional derivative of f at A [tex]\vec{u}_{AB}[/tex] is 9,
[tex]D\vec{u}_{AB}f=f_{x} \cdot 1 + f_{y}\cdot 0\\f_{x} =9[/tex]
The given directional derivative of f at A [tex]\vec{u}_{AC}[/tex] is 2,
[tex]D\vec{u}_{AB}f=f_{x} \cdot 0 + f_{y}\cdot 1\\f_{y} =2[/tex]
The given directional derivative of f at A [tex]\vec{u}_{AD}[/tex] is
[tex]D\vec{u}_{AD}f=f_{x} \cdot \frac{3}{5} + f_{y}\cdot \frac{4}{5}[/tex]
[tex]D\vec{u}_{AD}f=9 \cdot \frac{3}{5} + 2\cdot \frac{4}{5}[/tex]
[tex]D\vec{u}_{AD}f= \frac{27+8}{5} =7[/tex]
The directional derivative of f at A in the direction of [tex]\vec{u}_{AD}[/tex] is 7.
