Question Details

Let, ƒ(x,y,z) = 4x2+7xy+3xz2 .The direction in which the function ƒ(x,y,z) increases most rapidly at point P= (1,0,2) is

Options

A

20î + 12 k̂

B

20î +7Ĵ+ 12 k̂

C

20î + 7 Ĵ

D

20î

Show Answer

Correct Answer :

Option B

20î +7Ĵ+ 12 k̂

Solution :

To find the direction in which a scalar function f(x,y,z) increases most rapidly at a given point, we need to calculate the gradient of the function, f, at that point.

The gradient vector of f(x,y,z) is given by:
f = f x i ̂ + f y j ̂ + f z k ̂

The given function is:
f ( x , y , z ) = 4 x 2 + 7 x y + 3 x z 2

Let us compute the first-order partial derivatives:
1. With respect to x:
f x = x ( 4 x 2 + 7 x y + 3 x z 2 ) = 8 x + 7 y + 3 z 2
2. With respect to y:
f y = y ( 4 x 2 + 7 x y + 3 x z 2 ) = 7 x
3. With respect to z:
f z = z ( 4 x 2 + 7 x y + 3 x z 2 ) = 6 x z

Now, we evaluate these partial derivatives at the given point P=(1,0,2), where x=1, y=0, and z=2:
( f x ) ( 1 , 0 , 2 ) = 8 ( 1 ) + 7 ( 0 ) + 3 ( 2 ) 2 = 8 + 0 + 12 = 20
( f y ) ( 1 , 0 , 2 ) = 7 ( 1 ) = 7
( f z ) ( 1 , 0 , 2 ) = 6 ( 1 ) ( 2 ) = 12

Substituting these values back into the gradient formula gives:
f ( 1 , 0 , 2 ) = 20 i ̂ + 7 j ̂ + 12 k ̂
This vector represents the direction of the maximum rate of increase of f at the point P.

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