Suppose f(x,y)=x/y, P=(0,−1) and v=3i+3j. A. Find the gradient of f. ∇f= ____i+____j Note: Your answers should be expressions of x and y; e.g. "3x - 4y" B. Find the gradient of f at the point P. (∇f)(P)= ____i+____j  Note: Your answers should be numbers C. Find the directional derivative of f at P in the direction of v. Duf=? Note: Your answer should be a number D. Find the maximum rate of change of f at P. maximum rate of change of f at P=? Note: Your answer should be a number E. Find the (unit) direction vector in which the maximum rate of change occurs at P. u= ____i+____j Note: Your answers should be numbers

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
Section: Chapter Questions
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Suppose f(x,y)=x/y, P=(0,−1) and v=3i+3j.

A. Find the gradient of f.
∇f= ____i+____j
Note: Your answers should be expressions of x and y; e.g. "3x - 4y"

B. Find the gradient of f at the point P.
(∇f)(P)= ____i+____j 
Note: Your answers should be numbers

C. Find the directional derivative of f at P in the direction of v.
Duf=?
Note: Your answer should be a number

D. Find the maximum rate of change of f at P.
maximum rate of change of f at P=?


Note: Your answer should be a number

E. Find the (unit) direction vector in which the maximum rate of change occurs at P.
u= ____i+____j
Note: Your answers should be numbers

Expert Solution
Step 1

  Given that  fx,y = xy ,  P  0, -1   and  v = 3 i + 3 j

(a)  The gradient of f is  grad f or  f = fx i +fy j

                             f = 1y i - xy2 j

(b)   The gradient of f at the point  P  0, -1  is 

                       fP = 1-1 i - 0-12 j

              fP = - i 

            Answer :  fP = -1 i +0 j

(c)  The Unit vector in the direction of the vector v = 3 i + 3 j  is  

                         n = vv = 3 i+j312+12

              n=12 i+j

 Now the directional derivative of f in the direction of the vector v at the point P is

                       fP . n  = -i . 12 i+j

                                        =12 -11+01

                                     = - 12

            The directional derivative of f in the direction of the vector v at the point P is = - 12

              Answer :  Duf = -12

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