Calculus 3 : Partial Derivatives

Study concepts, example questions & explanations for Calculus 3

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Example Questions

Example Question #391 : Partial Derivatives

Find \(\displaystyle \frac{\partial f}{dx}\).

\(\displaystyle f(x,y,z)=xyz^{10}+\frac{xy}{\ln(yz)}+10e^z+e^{xy}\)

Possible Answers:

\(\displaystyle \frac{\partial f}{dx}=xz^{10}+\frac{1}{\ln(yz)}+xe^{xy}\)

\(\displaystyle \frac{\partial f}{dx}=xyz^{10}+\frac{xy}{\ln(yz)}+ye^{xy}\)

\(\displaystyle \frac{\partial f}{dx}=yz^{10}+\frac{y}{\ln(yz)}\)

\(\displaystyle \frac{\partial f}{dx}=\frac{y}{\ln(yz)}+ye^{xy}\)

\(\displaystyle \frac{\partial f}{dx}=yz^{10}+\frac{y}{\ln(yz)}+ye^{xy}\)

Correct answer:

\(\displaystyle \frac{\partial f}{dx}=yz^{10}+\frac{y}{\ln(yz)}+ye^{xy}\)

Explanation:

In order to find \(\displaystyle \frac{\partial f}{dx}\), we need to take the derivative of \(\displaystyle f\) in respect to  \(\displaystyle x\), and treat \(\displaystyle y\), and \(\displaystyle z\) as constants. We also need to remember what the derivatives of natural log, exponential functions and power functions are for single variables.

Natural Log:

\(\displaystyle f(x)=\ln(g(x))\)

\(\displaystyle f'(x)=\frac{g'(x)}{g(x)}\)

Exponential Functions:

\(\displaystyle f(x)=e^{g(x)}\)

\(\displaystyle f'(x)=g'(x)e^{g(x)}\)

Power Functions:

\(\displaystyle f(x)=x^n\)

\(\displaystyle f'(x)=nx^{n-1}\)

\(\displaystyle \frac{\partial f}{dx}=yz^{10}+\frac{y}{\ln(yz)}+0+ye^{xy}\)

\(\displaystyle \frac{\partial f}{dx}=yz^{10}+\frac{y}{\ln(yz)}+ye^{xy}\)

Example Question #392 : Partial Derivatives

True or False

\(\displaystyle \frac{\partial}{\partial y}\Big(x^x+wz\ln(uvxw)\Big)=0\)

Possible Answers:

False

True

Correct answer:

True

Explanation:

True:

Since there are no \(\displaystyle y's\) in the equation, the derivative of a constant is \(\displaystyle 0\).

Example Question #391 : Partial Derivatives

What is the partial derivative \(\displaystyle f_x(x,y)\) of the function

\(\displaystyle f(x,y)=e^{xy}\)?

Possible Answers:

\(\displaystyle f_x(x,y)=ye^{xy}\)

\(\displaystyle f_x(x,y)=xe^{xy}\)

\(\displaystyle f_x(x,y)=xye^{xy}\)

\(\displaystyle f_x(x,y)=e^{xy}\)

Correct answer:

\(\displaystyle f_x(x,y)=ye^{xy}\)

Explanation:

We can find \(\displaystyle f_x(x,y)\) given \(\displaystyle f(x,y)=e^{xy}\) by differentiating the function while holding \(\displaystyle y\) constant, i.e. we treat \(\displaystyle y\) as a number. So we get

\(\displaystyle f_x(x,y)=\frac{\partial}{\partial x}e^{xy}=e^{xy}\frac{\partial}{\partial x}xy=e^{xy}\cdot y=ye^{xy}\)

Example Question #392 : Partial Derivatives

Find the partial derivative \(\displaystyle f_y(x,y)\) for the function \(\displaystyle f(x,y)=2xy+2\).

Possible Answers:

\(\displaystyle f_y(x,y)=2y\)

\(\displaystyle f_y(x,y)=0\)

\(\displaystyle f_y(x,y)=2xy\)

\(\displaystyle f_y(x,y)=2x\)

Correct answer:

\(\displaystyle f_y(x,y)=2x\)

Explanation:

We can find \(\displaystyle f_y(x,y)\) from the function \(\displaystyle f(x,y)=2xy+2\) by taking the derivative holding \(\displaystyle x\) constant and letting \(\displaystyle y\) be the variable. This means

\(\displaystyle f_y(x,y)=\frac{\partial}{\partial y}(2xy+2)=2x\)

Example Question #393 : Partial Derivatives

Given the function \(\displaystyle f(x,y)=\sin x^2y\), find the partial derivative \(\displaystyle f_x(x,y)\).

Possible Answers:

\(\displaystyle f_x(x,y)=2x\cos x^2y\)

\(\displaystyle f_x(x,y)=y\cos x^2y\)

\(\displaystyle f_x(x,y)=2xy\cos x^2y\)

\(\displaystyle f_x(x,y)=x^2y\cos x^2y\)

Correct answer:

\(\displaystyle f_x(x,y)=2xy\cos x^2y\)

Explanation:

To find the partial derivative \(\displaystyle f_x(x,y)\) of the function \(\displaystyle f(x,y)=\sin x^2y\), we calculate the derivative of \(\displaystyle f(x,y)\) while treating \(\displaystyle y\) as a constant. So we get 

\(\displaystyle f_x(x,y)=\frac{\partial}{\partial x}f(x,y)=\frac{\partial}{\partial x}\sin x^2y=(\cos x^2y)\cdot \frac{\partial}{\partial x}x^2y\)

\(\displaystyle =2xy\cos x^2y\)

Example Question #394 : Partial Derivatives

Given the function \(\displaystyle f(x,y)=\sin x^2y\), find the partial derivative \(\displaystyle f_y(x,y)\).

Possible Answers:

\(\displaystyle f_y(x,y)=2xy\cos x^2y\)

\(\displaystyle f_y(x,y)=y\cos x^2y\)

\(\displaystyle f_y(x,y)=2x\cos x^2y\)

\(\displaystyle f_y(x,y)=y\cos x^2y\)

\(\displaystyle f_y(x,y)=x^2\cos x^2y\)

\(\displaystyle f_y(x,y)=2xy\cos x^2y\)

Correct answer:

\(\displaystyle f_y(x,y)=x^2\cos x^2y\)

Explanation:

To find the partial derivative \(\displaystyle f_y(x,y)\) of the function \(\displaystyle f(x,y)=\sin x^2y\), we calculate the derivative of \(\displaystyle f(x,y)\) while treating \(\displaystyle x\) as a constant. So we get 

\(\displaystyle f_y(x,y)=\frac{\partial}{\partial y}f(x,y)=\frac{\partial}{\partial y}\sin x^2y=(\cos x^2y)\cdot \frac{\partial}{\partial y}x^2y\)

\(\displaystyle =x^2\cos x^2y\)

Example Question #395 : Partial Derivatives

Given the function \(\displaystyle f(x,y)=\ln (1+x^2+y^2)\), find the partial derivative \(\displaystyle f_x(x,y)\).

Possible Answers:

\(\displaystyle f_x(x,y)=0\)

\(\displaystyle f_x(x,y)=\frac{2x}{1+x^2+y^2}\)

\(\displaystyle f_x(x,y)=\frac{2y}{1+x^2+y^2}\)

\(\displaystyle f_x(x,y)=\frac{2x+2y}{1+x^2+y^2}\)

Correct answer:

\(\displaystyle f_x(x,y)=\frac{2x}{1+x^2+y^2}\)

Explanation:

To find the partial derivative \(\displaystyle f_x(x,y)\) of the function \(\displaystyle f(x,y)=\ln (1+x^2+y^2)\), we differentiate the function with respect to \(\displaystyle x\), which means we hold \(\displaystyle y\) constant. Then we get (using the chain rule):

\(\displaystyle f_x(x,y)=\frac{\partial}{\partial x}f(x,y)=\frac{\partial}{\partial x}\ln (1+x^2+y^2)=\frac{1}{1+x^2+y^2}\cdot \frac{\partial}{\partial x}(1+x^2+y^2)\)

\(\displaystyle =\frac{2x}{1+x^2+y^2}\)

Example Question #396 : Partial Derivatives

Given the function \(\displaystyle f(x,y)=\ln (2+x^2+y^2)\), find the partial derivative \(\displaystyle f_y(x,y)\).

Possible Answers:

\(\displaystyle f_y(x,y)=0\)

\(\displaystyle f_y(x,y)=\frac{2x}{2+x^2+y^2}\)

\(\displaystyle f_y(x,y)=\frac{2x+2y}{1+x^2+y^2}\)

\(\displaystyle f_y(x,y)=\frac{2y}{1+x^2+y^2}\)

Correct answer:

\(\displaystyle f_y(x,y)=\frac{2y}{1+x^2+y^2}\)

Explanation:

To find the partial derivative \(\displaystyle f_y(x,y)\) of the function \(\displaystyle f(x,y)=\ln (2+x^2+y^2)\), we differentiate the function with respect to \(\displaystyle y\), which means we hold \(\displaystyle x\) constant. Then we get (using the chain rule):

\(\displaystyle f_y(x,y)=\frac{\partial}{\partial y}f(x,y)=\frac{\partial}{\partial y}\ln (2+x^2+y^2)=\frac{1}{2+x^2+y^2}\cdot \frac{\partial}{\partial y}(2+x^2+y^2)\)

\(\displaystyle =\frac{2y}{2+x^2+y^2}\)

Example Question #397 : Partial Derivatives

Given the function \(\displaystyle f(x,y)=e^{24xy+25x^2}\), find the partial derivative

\(\displaystyle f_x(x,y)\)

Possible Answers:

\(\displaystyle f_x(x,y)=(24y+50x)e^{24xy+25x^2}\)

\(\displaystyle f_x(x,y)=24xe^{24xy+25x^2}\,\)

\(\displaystyle f_x(x,y)=e^{24y+50x}\)

\(\displaystyle f_x(x,y)=(24y+50x)e^{24y+50x}\)

Correct answer:

\(\displaystyle f_x(x,y)=(24y+50x)e^{24xy+25x^2}\)

Explanation:

To find the partial derivative \(\displaystyle f_x\) of \(\displaystyle f(x,y)=e^{24xy+25x^2}\), we need to differentiate \(\displaystyle f(x,y)\) with respect to \(\displaystyle x\) while holding \(\displaystyle y\) constant. We can use the chain rule to get

\(\displaystyle f_x(x,y)=\frac{\partial}{\partial x}e^{24xy+25x^2}=e^{24xy+25x^2}\cdot \frac{\partial}{\partial x}(24xy+25x^2)\)

\(\displaystyle =(24y+50x)e^{24xy+25x^2}\)

Example Question #398 : Partial Derivatives

Given the function \(\displaystyle f(x,y)=e^{24xy+25x^2}\), find the partial derivative

\(\displaystyle f_y(x,y)\)

Possible Answers:

\(\displaystyle f_y(x,y)=24xe^{24xy+25x^2}\)

\(\displaystyle f_y(x,y)=e^{24y+50x}\)

\(\displaystyle f_y(x,y)=(24y+50x)e^{24xy+25x^2}\,\)

\(\displaystyle f_y(x,y)=(24y+50x)e^{24y+50x}\)

Correct answer:

\(\displaystyle f_y(x,y)=24xe^{24xy+25x^2}\)

Explanation:

To find the partial derivative \(\displaystyle f_y\) of \(\displaystyle f(x,y)=e^{24xy+25x^2}\), we need to differentiate \(\displaystyle f(x,y)\) with respect to \(\displaystyle y\) while holding \(\displaystyle x\) constant. We can use the chain rule to get

\(\displaystyle f_y(x,y)=\frac{\partial}{\partial y}e^{24xy+25x^2}=e^{24xy+25x^2}\cdot \frac{\partial}{\partial y}(24xy+25x^2)\)

\(\displaystyle =24xe^{24xy+25x^2}\)

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