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In Exercises 21-28, find the directional derivative of the given

function at the specified point Pand in the direction of the

given unit vector u.

localid="1650380514741" f(x,y)=xy2atP=(2,1),u=513i+1213j

Short Answer

Expert verified

The directional derivative of the given

function is4313

Step by step solution

01

Given data

f(x,y)=xy2

P=x0,y0=(2,1)andu=(i+j)=513i+1213j

02

Solution 

Consider directional derivative

Dufx0,y0=Limh0fx0+h,y0+hfx0,y0h

Duf(2,1)=Limh0f2513h,1+1213hf(2,1)h

Therefore

f2513h,1+1213h=2513h1+1213h2

2513h2413h+144169h2

=265h132413h+144169h2

=265h131+2413h+144169h2

=265h13169+312h+144h2169

=33865h169+312h+144h2

And

fx0,y0=f(2,1)=212=2

03

Substitute

Substituting

Dwf(2,1)=Limh033865h169+312h+144h2(2)h

=Limh033865h+338+624h+288h2h169+312h+144h2

=Limh0559h+288h2h169+312h+144h2

=Limh0h(559+288h)h169+312h+144h2

=Limh0(559+288h)169+312h+144h2=559169
Dwf(2,1)=4313

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