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Multivariable Calculus

8th Edition
James Stewart
ISBN: 9781305266643

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Multivariable Calculus

8th Edition
James Stewart
ISBN: 9781305266643
Textbook Problem

Find fx and fy and graph f, fx, and fy with domains and viewpoints that enable you to see the relationships between them.

14. f ( x , y ) = y 1 + x 2 y 2

To determine

To find: The local maximum, local minimum and saddle point of the function, f(x,y)=ycosx .

Explanation

Result used:

Second Derivative Test:

“Suppose the second partial derivatives of f are continuous on a disk with center (a,b) , and suppose that fx(a,b)=0 and fy(a,b)=0 (that is (a,b) is a critical point of f). Let

D=D(a,b)=fxx(a,b)fyy(a,b)[fxy(a,b)]2

(a) If D>0 and fxx(a,b)>0 , then f(a,b) is a local minimum.

(b) If D>0 and fxx(a,b)<0 , then f(a,b) is a local maximum.

(c) If D<0 , then f(a,b) is not a local maximum or minimum and it is called a saddle point”.

Calculation:

The given function is, f(x,y)=ycosx .

Take the partial derivative in the given function with respect to x and obtain fx .

fx=x(ycosx)=yx(cosx)=y(sinx)=ysinx

Thus, fx=ysinx (1)

Take the partial derivative in the given function with respect to y and obtain fy .

fy=y(ycosx)=cosxy(y)=cosx(1)=cosx

Thus, fy=cosx (2)

Set the above partial derivatives to 0 and find the values of x and y.

From the equation (1),

ysinx=0y=0,sinx0

From the equation (2),

cosx=0

The values of both cosx and sinx cannot be zero for the same x value. Thus, it is not possible.

So, take x=π2+nπ where n is an integer, n .

Thus, the critical points is, (x,y)=(π2+nπ,0) .

Take the partial derivative of the equation (1) with respect to x and obtain fxx

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Chapter 14 Solutions

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P-14RECh-14 P-15RECh-14 P-16RECh-14 P-17RECh-14 P-18RECh-14 P-19RECh-14 P-20RECh-14 P-21RECh-14 P-22RECh-14 P-23RECh-14 P-24RECh-14 P-25RECh-14 P-26RECh-14 P-27RECh-14 P-28RECh-14 P-29RECh-14 P-30RECh-14 P-31RECh-14 P-32RECh-14 P-33RECh-14 P-34RECh-14 P-35RECh-14 P-36RECh-14 P-37RECh-14 P-38RECh-14 P-39RECh-14 P-40RECh-14 P-41RECh-14 P-42RECh-14 P-43RECh-14 P-44RECh-14 P-45RECh-14 P-46RECh-14 P-47RECh-14 P-48RECh-14 P-49RECh-14 P-50RECh-14 P-51RECh-14 P-52RECh-14 P-53RECh-14 P-54RECh-14 P-55RECh-14 P-56RECh-14 P-57RECh-14 P-58RECh-14 P-59RECh-14 P-60RECh-14 P-61RECh-14 P-62RECh-14 P-63RECh-14 P-64RECh-14 P-65RECh-14 P-1PCh-14 P-2PCh-14 P-3PCh-14 P-4PCh-14 P-5PCh-14 P-6PCh-14 P-7PCh-14 P-8P

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