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Calculus 3 : Applications of Partial Derivatives

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Example Questions

Example Question #42 : Gradient Vector, Tangent Planes, And Normal Lines

Find $\bigtriangledown f$ of the following function:

$f(x, y, z)=xz^3\cos(y)$

Possible Answers:

$\left \langle z^3\cos(y), -xz^3\sin(y), 3xz^2\cos(y)\right \rangle$

$\left \langle z^3\cos(y), xz^3\sin(y), 3xz^2\cos(y)\right \rangle$

$\left \langle z^3\cos(y), -xz^3\sin(y), 3xz^2\sin(y)\right \rangle$

$\left \langle 0, -\sin(y), z^2\right \rangle$

Correct answer:

$\left \langle z^3\cos(y), -xz^3\sin(y), 3xz^2\cos(y)\right \rangle$

Explanation:

The gradient of the function is

$\bigtriangledown f= \left \langle f_x, f_y, f_z\right \rangle$

To find the given partial derivative of the function, we must treat the other variable(s) as constants.

The partial derivatives are

$f_x=z^3\cos(y)$

$f_y=-xz^3\sin(y)$

$f_z=3xz^2\cos(y)$

The derivatives were found using the following rules:

$\frac{\mathrm{d} }{\mathrm{d} x} ax=a$ , $\frac{\mathrm{d} }{\mathrm{d} x} \cos(x)=-\sin(x)$ , $\frac{\mathrm{d} }{\mathrm{d} x} x^n=nx^{n-1}$

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Example Question #43 : Gradient Vector, Tangent Planes, And Normal Lines

Find $\bigtriangledown f$ for the following function:

$f(x, y, z)=4e^{xy}+\ln(z)$

Possible Answers:

$\left \langle 4xe^{xy}, 4ye^{xy}, \frac{1}{z}\right \rangle$

$\left \langle 4ye^{xy}, 4xe^{xy}, \frac{1}{z}\right \rangle$

$\left \langle ye^{xy}, xe^{xy}, \frac{1}{z}\right \rangle$

$\left \langle 4ye^{xy}, 4xe^{xy}, -\frac{1}{z}\right \rangle$

Correct answer:

$\left \langle 4ye^{xy}, 4xe^{xy}, \frac{1}{z}\right \rangle$

Explanation:

The gradient of a function is given by

$\bigtriangledown f=\left \langle f_x, f_y, f_z\right \rangle$

To find the given partial derivative of the function, we must treat the other variable(s) as constants.

The partial derivatives are

$f_x=4ye^{xy}$

$f_y=4xe^{xy}$

$f_z=\frac{1}{z}$

The rules used to find the derivatives are

$\frac{\mathrm{d} }{\mathrm{d} x} e^u=e^u \frac{\mathrm{du} }{\mathrm{d} x}$ , $\frac{\mathrm{d} }{\mathrm{d} x} ax=a$ , $\frac{\mathrm{d} }{\mathrm{d} x} \ln(x)=\frac{1}{x}$

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Example Question #44 : Gradient Vector, Tangent Planes, And Normal Lines

Find $\bigtriangledown f$ of the following function:

$f(x, y, z)=z^2+\sec(xy)$

Possible Answers:

$\left \langle x\sec(xy)\tan(xy), y\sec(xy)\tan(xy), 2z\right \rangle$

$(y+x)(\sec(xy)\tan(xy)) +2z$

$\left \langle -y\sec(xy)\tan(xy), -x\sec(xy)\tan(xy), 2z\right \rangle$

$\left \langle y\sec(xy)\tan(xy), x\sec(xy)\tan(xy), 2z\right \rangle$

Correct answer:

$\left \langle y\sec(xy)\tan(xy), x\sec(xy)\tan(xy), 2z\right \rangle$

Explanation:

The gradient of the function is given by

$\bigtriangledown f=\left \langle f_x, f_y, f_z\right \rangle$

To find the given partial derivative of the function, we must treat the other variable(s) as constants.

The partial derivatives are

$f_x=y\sec(xy)\tan(xy)$

$f_y=x\sec(xy)\tan(xy)$

f_z=2z

The derivatives were found using the following rules:

$\frac{\mathrm{d} }{\mathrm{d} x} f(g(x))=f'(g(x))\cdot g'(x)$ , $\frac{\mathrm{d} }{\mathrm{d} x} \sec(x)=\sec(x)\tan(x)$ , $\frac{\mathrm{d} }{\mathrm{d} x} ax=a$ , $\frac{\mathrm{d} }{\mathrm{d} x} x^n=nx^{n-1}$

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Example Question #45 : Gradient Vector, Tangent Planes, And Normal Lines

Find $\nabla f$ of the given function:

$f(x, y, z)=xz^2\cos(y^3)$

Possible Answers:

$\left \langle z^2\cos(y^3), xz^2\sin(y^3), 2xz\cos(y^3)\right \rangle$

$\left \langle z^2\cos(y^3), 3xy^2z^2\sin(y^3), 2xz\cos(y^3)\right \rangle$

$\left \langle 0, 0, 0\right \rangle$

$\left \langle z^2\cos(y^3), -3xy^2z^2\sin(y^3), 2xz\cos(y^3)\right \rangle$

Correct answer:

$\left \langle z^2\cos(y^3), -3xy^2z^2\sin(y^3), 2xz\cos(y^3)\right \rangle$

Explanation:

The gradient of the function is given by

$\nabla f=\left \langle f_x, f_y, f_z\right \rangle$

To find the given partial derivative of the function, we must treat the other variable(s) as constants.

The partial derivatives are

$f_x=z^2\cos(y^3)$

$f_y=-3xy^2z^2\sin(y^3)$

$f_z=2xz\cos(y^3)$

The derivatives were found using the following rules:

$\frac{\mathrm{d} }{\mathrm{d} x} ax=a$ , $\frac{\mathrm{d} }{\mathrm{d} x} x^n=nx^{n-1}$ , $\frac{\mathrm{d} }{\mathrm{d} x} \cos(x)=-\sin(x)$

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Example Question #46 : Gradient Vector, Tangent Planes, And Normal Lines

Find $\nabla f$ of the following function:

$f(x, y, z)=e^x\tan(z)+y^2z^4$

Possible Answers:

$\left \langle e^x, 2y, 2z^3\right \rangle$

$\left \langle e^x\tan(z), 2yz^4, 4y^2z^3\right \rangle$

$\left \langle e^x\tan(z), yz^4, y^2z^3\right \rangle$

$e^x\tan(z) + 2yz^4+4y^2z^3$

Correct answer:

$\left \langle e^x\tan(z), 2yz^4, 4y^2z^3\right \rangle$

Explanation:

The gradient of the function is given by

$\nabla f=\left \langle f_x, f_y, f_z\right \rangle$

To find the given partial derivative of the function, we must treat the other variable(s) as constants.

The partial derivatives are

$f_x=e^x\tan(z)$

f_y=2yz^4

f_z=4y^2z^3

The derivatives were found using the following rules:

$\frac{\mathrm{d} }{\mathrm{d} x} ax=a$ , $\frac{\mathrm{d} }{\mathrm{d} x} x^n=nx^{n-1}$ , $\frac{\mathrm{d} }{\mathrm{d} x} e^x=e^x$

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Example Question #51 : Gradient Vector, Tangent Planes, And Normal Lines

Find the equation of the tangent plane to the following function at (2, 5, 4) :

f(x, y, z)=xy+4z

Possible Answers:

5x+2y+4z=36

2x+5y+4z=36

5x+2y+4z=0

2x+5y+4z=0

Correct answer:

5x+2y+4z=36

Explanation:

The equation of the tangent plane is given by

f_x(x_0, y_0, z_0)(x-x_0)+f_y(x_0, y_0, z_0)(y-y_0)+f_z(x_0, y_0, z_0)(z-z_0)=0

So, we must find the partial derivatives for the function evaluated at the point given. To find the given partial derivative of the function, we must treat the other variable(s) as constants.

The partial derivatives are

f_x=y

f_y=x

f_z=4

The derivatives were found using the following rule:

$\frac{\mathrm{d} }{\mathrm{d} x} ax=a$

Evaluated at the given point, the partial derivatives are

f_x=5

f_y=2

f_z=4

Note that the partial derivative with respect to z was 4 to begin with; the fact that the point has a z coordinate of 4 is a coincidence.

Now, plug all of this into our given formula:

5(x-2)+2(y-5)+4(z-4)=0

which simplified becomes

5x+2y+4z=36

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Example Question #52 : Gradient Vector, Tangent Planes, And Normal Lines

Find $\nabla f$ of the following function:

f(x, y, z)=ze^z+xy

Possible Answers:

$\left \langle y, x, 2e^z\right \rangle$

$\left \langle x, y, e^z+ze^z\right \rangle$

$\left \langle y, x, e^z\right \rangle$

$\left \langle y, x, e^z+ze^z\right \rangle$

Correct answer:

$\left \langle y, x, e^z+ze^z\right \rangle$

Explanation:

The gradient of a function is given by

$\nabla f=\left \langle f_x, f_y, f_z\right \rangle$

To find the given partial derivative of the function, we must treat the other variable(s) as constants.

The partial derivatives are

f_x=y

f_y=x

f_z=e^z+ze^z

The derivatives were found using the following rules:

$\frac{\mathrm{d} }{\mathrm{d} x} ax=a$ , $\frac{\mathrm{d} }{\mathrm{d} x} f(x)g(x)=f'(x)g(x)+f(x)g'(x)$ , $\frac{\mathrm{d} }{\mathrm{d} x} e^x=e^x$

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Example Question #53 : Gradient Vector, Tangent Planes, And Normal Lines

Find $\nabla f$ of the following function:

f(x, y, z)=xyz^4+ze^y

Possible Answers:

$\left \langle yz^4, xz^4+ze^y, 4xyz^3+e^y\right \rangle$

$\left \langle yz^4, xz^4+ze^y, 4xyz+ze^y\right \rangle$

yz^4+xz^4+4xyz^3+2ze^y

$\left \langle yz^4, xz^4+ze^y, 4xyz^3+ze^y\right \rangle$

Correct answer:

$\left \langle yz^4, xz^4+ze^y, 4xyz^3+e^y\right \rangle$

Explanation:

The gradient of a function is given by

$\nabla f=\left \langle f_x, f_y, f_z\right \rangle$

To find the given partial derivative of the function, we must treat the other variable(s) as constants.

The partial derivatives are

f_x=yz^4

f_y=xz^4+ze^y

f_z=4xyz^3+ze^y

The derivatives were found using the following rules:

$\frac{\mathrm{d} }{\mathrm{d} x} ax=a$ , $\frac{\mathrm{d} }{\mathrm{d} x} e^x=e^x$ , $\frac{\mathrm{d} }{\mathrm{d} x} x^n=nx^{n-1}$

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Example Question #54 : Gradient Vector, Tangent Planes, And Normal Lines

Find $\nabla f$ of the following function:

$f(x,y,z)=\sec(xyz^2)$

Possible Answers:

$\left \langle yz^2\sec(xyz^2)\tan(xyz^2), xz^2\sec(xyz^2)\tan(xyz^2), xyz\sec(xyz^2)\tan(xyz^2)\right \rangle$

$\left \langle yz^2\sec(xyz^2)\tan(xyz^2), xz^2\sec(xyz^2)\tan(xyz^2), 2xyz^2\sec(xyz^2)\tan(xyz^2)\right \rangle$

$\left \langle x\sec(xyz^2)\tan(xyz^2), y\sec(xyz^2)\tan(xyz^2), 2z\sec(xyz^2)\tan(xyz^2)\right \rangle$

$\left \langle yz^2\sec(xyz^2)\tan(xyz^2), xz^2\sec(xyz^2)\tan(xyz^2), 2xyz\sec(xyz^2)\tan(xyz^2)\right \rangle$

Correct answer:

$\left \langle yz^2\sec(xyz^2)\tan(xyz^2), xz^2\sec(xyz^2)\tan(xyz^2), 2xyz\sec(xyz^2)\tan(xyz^2)\right \rangle$

Explanation:

The gradient of a function is given by

$\nabla f=\left \langle f_x, f_y, f_z\right \rangle$

To find the given partial derivative of the function, we must treat the other variable(s) as constants.

The partial derivatives are

$f_x=yz^2\sec(xyz^2)\tan(xyz^2)$

$f_y=xz^2\sec(xyz^2)\tan(xyz^2)$

$f_z=2xyz\sec(xyz^2)\tan(xyz^2)$

The derivatives were found using the following rules:

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Example Question #51 : Gradient Vector, Tangent Planes, And Normal Lines

Find the equation of the tangent plane to the given function at (2, 3, 0) :

f(x,y,z)=xyz+y^2z

Possible Answers:

x+y+z=0

z=0

$z=\frac{1}{6}$

z=6

Correct answer:

z=0

Explanation:

The equation of the tangent plane is given by

f_x(x_0, y_0, z_0)(x-x_0)+f_y(x_0, y_0, z_0)(y-y_0)+f_z(x_0, y_0, z_0)(z-z_0)=0

So, we must find the partial derivatives for the function evaluated at the point given. To find the given partial derivative of the function, we must treat the other variable(s) as constants.

The partial derivatives are

f_x=yz

f_y=xz+2yz^2

f_z=xy+2y^2z

The partial derivatives evaluated at the given point are

f_x=0

f_y=0

f_z=6

Plugging all of this into the above formula, we get

6(z-0)=0

z=0

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Calculus 3 : Applications of Partial Derivatives

Study concepts, example questions & explanations for Calculus 3

All Calculus 3 Resources

Example Questions

Example Question #42 : Gradient Vector, Tangent Planes, And Normal Lines

Example Question #43 : Gradient Vector, Tangent Planes, And Normal Lines

Example Question #44 : Gradient Vector, Tangent Planes, And Normal Lines

Example Question #45 : Gradient Vector, Tangent Planes, And Normal Lines

Example Question #46 : Gradient Vector, Tangent Planes, And Normal Lines

Example Question #51 : Gradient Vector, Tangent Planes, And Normal Lines

Example Question #52 : Gradient Vector, Tangent Planes, And Normal Lines

Example Question #53 : Gradient Vector, Tangent Planes, And Normal Lines

Example Question #54 : Gradient Vector, Tangent Planes, And Normal Lines

Example Question #51 : Gradient Vector, Tangent Planes, And Normal Lines

All Calculus 3 Resources

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