Calculus 1 : Functions

Study concepts, example questions & explanations for Calculus 1

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Example Questions

Example Question #111 : How To Find Differential Functions

\(\displaystyle f(x)=(3x^{2}+2x)^{3}\)

Find

\(\displaystyle \frac{d}{dx}f(x)\).

Possible Answers:

\(\displaystyle 3(6x+2)^2\)

\(\displaystyle (6x+2)^3\)

\(\displaystyle (18x+6)(3x^2+2x)^2\)

\(\displaystyle 3(3x^2+2x)^2\)

\(\displaystyle (3x^2+2x)^3(6x+2)\)

Correct answer:

\(\displaystyle (18x+6)(3x^2+2x)^2\)

Explanation:

Let \(\displaystyle g(x)=3x^2+2x\).

Then \(\displaystyle f(x) = g(x)^3\).

By the chain rule,

\(\displaystyle \frac{df}{dx}=\frac{df}{dg}\frac{dg}{dx}\)

\(\displaystyle \frac{df}{dg}=3g(x)^2\)\(\displaystyle \frac{dg}{dx}=6x+2\)

Plugging everything in we get

\(\displaystyle \frac{df}{dx}=3(3x^2+2x)^2(6x+2)=(18x+6)(3x^2+2x)^2\)

 

Example Question #1331 : Calculus

Let \(\displaystyle f(x)=\frac{x^2}{e^x+3x^3}\)

Find 

\(\displaystyle \frac{d}{dx}f(x)\).

Possible Answers:

\(\displaystyle \frac{(e^x+3x^3)^2-x^2(e^x+9x^2)}{(e^x+3x^3)^2}\)

\(\displaystyle \frac{(e^x+3x^3)-x^2(e^x+9x^2)}{(e^x+3x^3)}\)

\(\displaystyle \frac{2x(e^x+3x^3)-x^2(e^x+9x^2)}{(e^x+3x^3)}\)

\(\displaystyle \frac{2x(e^x+3x^3)-x^2(e^x+9x^2)}{(e^x+3x^3)^2}\)

\(\displaystyle \frac{(e^x+3x^3)-x^2(e^x+9x^2)}{(e^x+3x^3)^2}\)

Correct answer:

\(\displaystyle \frac{2x(e^x+3x^3)-x^2(e^x+9x^2)}{(e^x+3x^3)^2}\)

Explanation:

Let \(\displaystyle g(x)=x^2\) and \(\displaystyle h(x)=(e^x+ex^3)^{-1}\).

So \(\displaystyle f(x)=g(x)h(x)\).

By the product rule:

\(\displaystyle \frac{df}{dx}=g'h+h'g\)

Where \(\displaystyle h'=\frac{dh}{dx}\) and \(\displaystyle g'=\frac{dg}{dx}\).

Therefore,

\(\displaystyle g'=2x\)

\(\displaystyle h'=-(e^x+3x^3)^{-2}(e^x+9x^2)\)

Plugging everything in and simplifying we get:

\(\displaystyle \frac{df}{dx}=\frac{2x(e^x+3x^3)-x^2(e^x+9x^2)}{(e^x+3x^3)^2}\)

Example Question #111 : Other Differential Functions

Let \(\displaystyle f(x)=ln[x^3e^x]\)

Find 

\(\displaystyle \frac{d}{dx}f(x)\).

Possible Answers:

\(\displaystyle ln[x^3e^x]\)

\(\displaystyle \frac{1}{x^3e^x}\)

\(\displaystyle ln[x^3e^x]^{-1}(3x^2e^x+x^3e^x)\)

\(\displaystyle ln[3x^2e^x+x^3e^x]\)

\(\displaystyle \frac{3}{x}+1\)

Correct answer:

\(\displaystyle \frac{3}{x}+1\)

Explanation:

We can simplify the function by using the properties of logarithms.

\(\displaystyle f(x)=ln[x^3]+ln[e^x]=3ln[x]+x\)

With the simplified form, we can now find the derivative using the power rule which states,

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

Also we will need to use the product rule which is,

\(\displaystyle f(x)=g(x)h(x)\rightarrow f'(x)=g(x)h'(x)+h(x)g'(x)\).

Remember that the derivative of \(\displaystyle ln(x)\rightarrow \frac{1}{x}\).

Applying these rules we find the derivative to be as follows.

\(\displaystyle \frac{df}{dx}=\frac{d}{dx}[3ln[x]+x]=\frac{3}{x}+1\)

 

Example Question #301 : Functions

Let \(\displaystyle f(x)=3^x\).

Find 

\(\displaystyle \frac{d}{dx}f(x)\).

Possible Answers:

\(\displaystyle ln(3)3^x\)

\(\displaystyle (x-1)3^{x-1}\)

\(\displaystyle log_3(3)3^x\)

\(\displaystyle x3^{x-1}\)

\(\displaystyle ln(x)3^x\)

Correct answer:

\(\displaystyle ln(3)3^x\)

Explanation:

For a function of the form \(\displaystyle f(x)=a^x\) the derivative is by definition:

\(\displaystyle \frac{d}{dx}f(x)=f'(x)=ln(a)a^x\).

Therefore,

\(\displaystyle f(x)=3^x\rightarrow f'(x)=3^xln(x)\).

Example Question #1333 : Calculus

Let \(\displaystyle f(x)=sin^2(x)\)

Find 

\(\displaystyle \frac{d}{dx}f(x)\).

Possible Answers:

\(\displaystyle 2cos(x)sin(x)\)

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

\(\displaystyle cos(x)+sin(x)\)

\(\displaystyle 2sin(x)\)

\(\displaystyle sin^2(x)cos^2(x)\)

Correct answer:

\(\displaystyle 2cos(x)sin(x)\)

Explanation:

Recall that, 

\(\displaystyle sin^2(x)=sin(x)sin(x)\)

Using the product rule

\(\displaystyle \frac{d}{dx}sin^2(x)=cos(x)sin(x)+sin(x)cos(x)=2cos(x)sin(x)\)

Example Question #112 : How To Find Differential Functions

Compute the differential for the following.

\(\displaystyle y=9t^2+24t-50\)

Possible Answers:

\(\displaystyle dy=(18t+24)dt\)

\(\displaystyle dy=(18t+24t)dt\)

\(\displaystyle dy=(18t^2+24)dt\)

\(\displaystyle dy=(18t+24)^2dt\)

\(\displaystyle dy=(18t^4+24t^2-50t)dt\)

Correct answer:

\(\displaystyle dy=(18t+24)dt\)

Explanation:

To compute the differential of the function we will need to use the power rule which states,

\(\displaystyle y=x^n \rightarrow dy=nx^{n-1}\).

Applying the power rule we get: 

\(\displaystyle \frac{dy}{dt}=18t+24\)

From here solve for dy: 

\(\displaystyle dy=(18t+24)dt\)

Example Question #1334 : Calculus

Compute the differential for the following function.

\(\displaystyle y=7x^3+cos(x)\)

Possible Answers:

\(\displaystyle dy=(21x^2+cos(x))dx\)

\(\displaystyle dy=(21x^2-sin(x))dx\)

\(\displaystyle dy=(21x^2-cos(x))dx\)

\(\displaystyle dy=(21x^2+sin(x))dx\)

\(\displaystyle dy=(21x^2-sin(x)cos(x))dx\)

Correct answer:

\(\displaystyle dy=(21x^2-sin(x))dx\)

Explanation:

Using the power rule,

\(\displaystyle y=x^n \rightarrow dy=nx^{n-1}dx\)

the derivative of \(\displaystyle 7x^3\) becomes \(\displaystyle 21x^2\).

Using trigonometric identities, the derivative of \(\displaystyle cos(x)\) is \(\displaystyle -sin(x)\)

Therefore, 

\(\displaystyle \frac{dy}{dx}=21x^2-sin(x)\)

\(\displaystyle dy=(21x^2-sin(x))dx\)

Example Question #301 : Differential Functions

Compute the differential for the following.

\(\displaystyle y=sin(x)cos(x)\)

Possible Answers:

\(\displaystyle dy=[sin^2(x)+cos^2(x)]dx\)

\(\displaystyle dy=[-sin^2(x)+cos^2(x)]dx\)

\(\displaystyle dy=[-sin(x)+cos^2(x)]dx\)

\(\displaystyle dy=[sin^2(x)-cos^2(x)]dx\)

\(\displaystyle dy=cos^2(x)dx\)

Correct answer:

\(\displaystyle dy=[-sin^2(x)+cos^2(x)]dx\)

Explanation:

To solve this problem, you must use the product rule of finding derivatives.

For any function \(\displaystyle y=ab\), \(\displaystyle y'=a(b')+b(a')\).

In this problem, the product rule yields 

\(\displaystyle a=sin(x), a'=cos(x)\)

\(\displaystyle b=cos(x), b'=-sin(x)\)

\(\displaystyle \frac{dy}{dx}=sin(x)\cdot(-sin(x))+cos(x)\cdot cos(x)\)

\(\displaystyle dy=[-sin^2(x)+cos^2(x)]dx\).

Example Question #1334 : Calculus

Calculate the differential for the following function.

\(\displaystyle y=2+3x-x^2\)

Possible Answers:

\(\displaystyle dy=(3-2x)\)

\(\displaystyle dy=(-3-2x)dx\)

\(\displaystyle dy=(3-2x)dx\)

\(\displaystyle dy=(3x-2x)dx\)

\(\displaystyle dy=(-2x)dx\)

Correct answer:

\(\displaystyle dy=(3-2x)dx\)

Explanation:

This differential can be found by utilizing the power rule,

\(\displaystyle y=x^n \rightarrow dy=nx^{n-1}dx\).

The original equation is \(\displaystyle y=2+3x-x^2\).

Using the power rule on each term we see that the derivative of \(\displaystyle 2\) is \(\displaystyle 0\). The derivative of a constant is always zero.

The dervative of \(\displaystyle 3x\) is

\(\displaystyle 1\cdot3 x^{1-1}=3x^0=3\).

The derivative of \(\displaystyle -x^2\) is

\(\displaystyle -1\cdot 2x^{2-1}=-2x^1=-2x\).

Thus, 

\(\displaystyle \frac{dy}{dx}=3-2x\)

Multiply \(\displaystyle dx\) to the right side to get the final solution. 

\(\displaystyle dy=(3-2x)dx\)

Example Question #1335 : Calculus

Compute the following differential.

\(\displaystyle y=4x^3+8x^2+32x+\pi\)

Possible Answers:

\(\displaystyle dy=(12x^2-16x+32)dx\)

\(\displaystyle dy=(12x+16x+32)dx\)

\(\displaystyle dy=(12x^2+16x+32)dx\)

\(\displaystyle dy=(12x^2-16x-32)dx\)

\(\displaystyle dy=(12x^2+16x+32+\Pi )dx\)

Correct answer:

\(\displaystyle dy=(12x^2+16x+32)dx\)

Explanation:

Using the power rule, we can find the derivative of each part of the function. The power rule states to multiply the coefficient of the term by the exponent then decrease the exponent by one.

The derivative of \(\displaystyle 4x^3\) is \(\displaystyle 12x^2\).

The derivative of \(\displaystyle 8x^2\) is \(\displaystyle 16x\).

The derivative of \(\displaystyle 32x\) is \(\displaystyle 32\).

And finally, because \(\displaystyle \pi\) is a constant, the derivative of \(\displaystyle \pi\) is \(\displaystyle 0\).

Thus, when we add the parts together, the derivative is 

\(\displaystyle \frac{dy}{dx}=12x^2+16x+32\)

and

\(\displaystyle dy=(12x^2+16x+32)dx\)

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