Calculus 2 : Derivatives

Study concepts, example questions & explanations for Calculus 2

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Example Questions

Example Question #1121 : Calculus Ii

Evaluate the limit using one of the definitions of a derivative.

\(\displaystyle \lim_{x \rightarrow -\frac{\pi}{2}} \frac{\sin^2x -1}{x+\frac{\pi}{2}}\)

Possible Answers:

\(\displaystyle \pi\)

\(\displaystyle \frac{1}{2}\)

\(\displaystyle 1\)

\(\displaystyle 0\)

Does not exist

Correct answer:

\(\displaystyle 0\)

Explanation:

Evaluating the limit directly will produce an indeterminant solution of \(\displaystyle \small \tiny{\frac{0}{0}}\).

The limit definition of a derivative is \(\displaystyle \lim_{h \rightarrow 0} \frac{f(x+h)-f(x)}{h}=f'(x)\). However, the alternative form, \(\displaystyle \lim_{x \rightarrow a} \frac{f(x)-f(a)}{x-a}=f'(x)\), better suits the given limit.

Let \(\displaystyle f(x)=\sin^2x\) and notice \(\displaystyle f\left(-\frac{\pi}{2}\right)=1\). It follows that \(\displaystyle {\lim_{x \rightarrow -\frac{\pi}{2}} \frac{\sin^2-1}{x-\left(-\frac{\pi}{2} \right )} = \frac{d}{dx}\sin^2x}\).  

Thus, the limit is \(\displaystyle {2\sin\left(-\frac{\pi}{2} \right ) \cos\left(-\frac{\pi}{2}\right)} = 0.\)

Example Question #1122 : Calculus Ii

Evaluate the limit using one of the definitions of a derivative.

\(\displaystyle \lim_{x \rightarrow 1} \frac{x^{49}-1}{x-1}\)

Possible Answers:

Does not exist

\(\displaystyle 49\)

\(\displaystyle 48\)

\(\displaystyle 0\)

\(\displaystyle 1\)

Correct answer:

\(\displaystyle 49\)

Explanation:

Evaluating the derivative directly will produce an indeterminant solution of \(\displaystyle \small \tiny{\frac{0}{0}}\).

The limit definition of a derivative is \(\displaystyle \lim_{h \rightarrow 0} \frac{f(x+h)-f(x)}{h}=f'(x)\). However, the alternative form, \(\displaystyle \lim_{x \rightarrow a} \frac{f(x)-f(a)}{x-a}=f'(x)\), better suits the given limit.

Let \(\displaystyle f(x)=x^{49}\) and notice \(\displaystyle f(1)=1\). It follows that \(\displaystyle {\lim_{x \rightarrow 1} \frac{x^{49}-1}{x-1} = \frac{d}{dx}x^{49}}\).  Thus, the limit is \(\displaystyle 49(1)^{48}=49\).

 

 

Example Question #1 : Derivatives

Suppose \(\displaystyle f\) and \(\displaystyle g\) are differentiable functions, and \(\displaystyle f(3)=1, f'(3)=4, g(-1)=3, g'(-1)=-1\). Calculate the derivative of \(\displaystyle h(x) = f(x+2)g(-x)\), at \(\displaystyle x = 1\)

Possible Answers:

\(\displaystyle 0\)

\(\displaystyle 2\)

\(\displaystyle -8\)

\(\displaystyle 24\)

None of the other answers

Correct answer:

None of the other answers

Explanation:

The correct answer is 11.

Taking the derivative of \(\displaystyle h(x)\) involves the product rule, and the chain rule.

\(\displaystyle h'(x)=[f(x+2)][-g'(-x)]+[g(-x)][f'(x+2)]\)

\(\displaystyle = -g'(-x)f(x+2)+g(-x)f'(x+2)\)

Substituting \(\displaystyle x=1\) into both sides of the derivative we get

\(\displaystyle h'(1)= -g'(-1)f(3)+g(-1)f'(3) = -1\cdot1 + 3 \cdot 4 = 11\).

Example Question #4 : Definition Of Derivative

Evaluate the limit

\(\displaystyle \small \small \lim_{\mu\to 0} \frac{\tan \mu}{\mu}\)

without using L'Hopital's rule.

Possible Answers:

\(\displaystyle \small \frac{\sqrt{2}}{2}\)

\(\displaystyle \small -\frac{\sqrt{2}}{2}\)

\(\displaystyle \small 2\)

\(\displaystyle \small 1\)

Correct answer:

\(\displaystyle \small 1\)

Explanation:

If we recall the definition of a derivative of a function \(\displaystyle \small f(x)\) at a point \(\displaystyle \small x\), one of the definitions is 

\(\displaystyle \small f'(x)=\lim_{h\to 0}\frac{f(x+h)-f(x)}{h}\).

If we compare this definition to the limit 

\(\displaystyle \small \small \lim_{\mu\to 0} \frac{\tan \mu}{\mu}\)

we see that that this is the limit definition of a derivative, so we need to find the function \(\displaystyle \small f(x)\) and the point \(\displaystyle \small x\) at which we are evaluating the derivative at. It is easy to see that the function is \(\displaystyle \small f(x)=\tan x\) and the point is \(\displaystyle \small x=0\). So finding the limit above is equivalent to finding \(\displaystyle \small f'(0)\).

We know that the derivative is \(\displaystyle \small f'(x)=\sec^2(x)\), so we have

\(\displaystyle \small \small \small \lim_{\mu\to 0} \frac{\tan \mu}{\mu}=f'(0)=\sec^2(0)=1^2=1\).

Example Question #1 : Definition Of Derivative

Approximate the derivative if \(\displaystyle y=\sqrt{x}\) where \(\displaystyle x=2\).

Possible Answers:

\(\displaystyle 0.35359\)

\(\displaystyle 0.35353\)

\(\displaystyle 0.35355\)

\(\displaystyle 0.35346\)

\(\displaystyle 0.35351\)

Correct answer:

\(\displaystyle 0.35355\)

Explanation:

Write the definition of the limit.

\(\displaystyle \lim_{h\to 0}\frac{f(x+h)-f(x)}{h}\)

Substitute \(\displaystyle x=2\).

\(\displaystyle \lim_{h\to 0}\frac{f(2+h)-f(2)}{h}=\lim_{h\to 0}\frac{\sqrt{2+h}-\sqrt2}{h}\)

Since \(\displaystyle h\) is approaching to zero, it would be best to evaluate when we assume that \(\displaystyle h\) is progressively decreasing.  Let's assume \(\displaystyle h=0.1, 0.0001, 0.000001\) and check the pattern.

\(\displaystyle \frac{\sqrt{2+0.1}-\sqrt2}{0.1}=0.349241\)

\(\displaystyle \frac{\sqrt{2+0.0001}-\sqrt2}{0.0001}=0.353549\)

\(\displaystyle \frac{\sqrt{2+0.000001}-\sqrt2}{0.000001}=0.353553\)

The best answer is:  \(\displaystyle 0.35355\)

Example Question #2 : Definition Of Derivative

Given:

\(\displaystyle f(x)=3x^2(x^2-5x+2)^5\)

 Find f'(x):

Possible Answers:

\(\displaystyle (x^2-5x+2)^4(36x^3-45x^2+12x)\)

\(\displaystyle (x^2-5x+2)^5(36x^3-105x^2+12x)\)

\(\displaystyle (x^2-5x+2)^4(36x^3-105x^2+12x)\)

\(\displaystyle (15x^2)(x^2-5x+2)^4\)

\(\displaystyle (x^2-5x+2)^4(60x^2-150)\)

Correct answer:

\(\displaystyle (x^2-5x+2)^4(36x^3-105x^2+12x)\)

Explanation:

Computation of the derivative requires the use of the Product Rule and Chain Rule. 

The Product Rule is used in a scenario when one has two differentiable functions multiplied by each other:

\(\displaystyle f(x)*g(x)\)

\(\displaystyle \frac{d}{dx}[f(x)*g(x)]=f(x)*g'(x)+g(x)*f(x)\)

This can be easily stated in words as: "First times the derivative of the second, plus the second times the derivative of the first."

In the problem statement, we are given:

\(\displaystyle f(x)=3x^2(x^2-5x+2)^5\)

\(\displaystyle 3x^2\) is the "First" function, and \(\displaystyle (x^2-5x+2)^5\) is the "Second" function. 

The "Second" function requires use of the Chain Rule. 

When:

\(\displaystyle y=(f(x))^a \rightarrow \frac{dy}{dx}=a*(f(x))^{a-1}*f'(x)\)

Applying these formulas results in:

\(\displaystyle f'(x)=(3x^2)[5(x^2-5x+2)^4(2x-5)]+(x^2-5x+2)^5(6x)\)

Simplifying the terms inside the brackets results in:

\(\displaystyle f'(x)=(3x^2)[(10x-25)(x^2-5x+2)^4]+(x^2-5x+2)^5(6x)\)

We notice that there is a common term that can be factored out in the sets of equations on either side of the "+" sign. Let's factor these out, and make the equation look "cleaner".

\(\displaystyle f'(x)=(x^2-5x+2)^4[(3x^2)(10x-25)+(6x)(x^2-5x+2)]\)

Inside the brackets, it is possible to clean up the terms into one expanded function. Let us do this:

\(\displaystyle f'(x)=(x^2-5x+2)^4[30x^3-75x^2+6x^3-30x^2+12x]\)

Simplifying this results in one of the answer choices:

\(\displaystyle f'(x)=(x^2-5x+2)^4(36x^3-105x^2+12x)\)

Example Question #2 : Derivatives

What is the value of the limit below?

\(\displaystyle \lim_{x\to \frac{\pi}{6}}\frac{sin(x)-sin(\frac{\pi}{6})}{x-\frac{\pi}{6}}\)

Possible Answers:

\(\displaystyle 0\)

\(\displaystyle \frac{\sqrt3}{2}\)

\(\displaystyle 1\)

\(\displaystyle \frac{1}{2}\)

Correct answer:

\(\displaystyle \frac{\sqrt3}{2}\)

Explanation:

Recall that one definition for the derivative of a function \(\displaystyle f(x)\ at\ x=a\) is \(\displaystyle f'(a)=\lim_{x\to a}\frac{f(x)-f(a)}{x-a}\).

This means that this question is asking us to find the value of the derivative of \(\displaystyle sin(x)\) at \(\displaystyle x=\frac{\pi}{6}\).

Since 

\(\displaystyle \frac{d}{dx}sin(x)=cos(x)\) and \(\displaystyle cos(\frac{\pi}{6})=\frac{\sqrt3}{2}\), the value of the limit is \(\displaystyle \frac{\sqrt3}{2}\).

Example Question #3 : Derivatives

\(\displaystyle f(x)=x^2\tan^{-1}(3x^5)\)

\(\displaystyle \frac{dy}{dx}=?\)

Possible Answers:

\(\displaystyle 2x\tan^{-1}(3x^5)\)

\(\displaystyle \frac{15x^6}{1+9x^{10}}+2x\tan^{-1}(3x^5)\)

\(\displaystyle \frac{15x^4\tan^{-1}(3x^5)}{1+9x^{7}}+2x^3\)

\(\displaystyle \frac{15x^8}{1+9x^7}+2x\tan^{-1}(3x^5)\)

\(\displaystyle \frac{15x^4\tan^{-1}(3x^5)}{1+9x^{10}}+2x^3\)

Correct answer:

\(\displaystyle \frac{15x^6}{1+9x^{10}}+2x\tan^{-1}(3x^5)\)

Explanation:

Evaluation of this integral requires use of the Product Rule. One must also need to recall the form of the derivative of \(\displaystyle \tan^{-1}(x)\).

Product Rule:

 \(\displaystyle \frac{d}{dx}[f(x)g(x)]=f(x)g'(x)+g(x)f'(x)\)

\(\displaystyle \frac{d}{dx}[\tan^{-1}(u)]=\frac{1}{1+u^2}\frac{du}{dx}\)

Applying these two rules results in:

\(\displaystyle \frac{dy}{dx}=\frac{d}{dx}[x^2\tan^{-1}(3x^5)]\)

\(\displaystyle \frac{dy}{dx}=(x^2)(\frac{1}{1+(3x^5)^2})(15x^4)+\tan^{-1}(3x^5)(2x)\)

\(\displaystyle \frac{dy}{dx}=\frac{15x^6}{1+9x^{10}}+2x\tan^{-1}(3x^5)\)

This matches one of the answer choices.

 

 

 

Example Question #1 : Definition Of Derivative

Use the definition of the derivative to solve for \(\displaystyle f'(x)\).

\(\displaystyle f(x)=x^3\)

Possible Answers:

\(\displaystyle 0\)

\(\displaystyle 3x^2\)

\(\displaystyle 1\)

\(\displaystyle x^2\)

\(\displaystyle \frac{1}{3}x^2\)

Correct answer:

\(\displaystyle 3x^2\)

Explanation:

In order to find \(\displaystyle f'(x)\), we need to remember how to find \(\displaystyle f'(x)\) by using the definition of derivative.

Definition of Derivative:

\(\displaystyle f'(x)=\lim_{h\rightarrow0} \frac{f(x+h)-f(x)}{h}\)

Now lets apply this to our problem.

 

\(\displaystyle f'(x)=\lim_{h\rightarrow0}\frac{(x+h)^3-x^3}{h}\)

Now lets expand the numerator.

 \(\displaystyle f'(x)=\lim_{h\rightarrow0}\frac{x^3+3x^2h+3xh^2+h^3-x^3}{h}\)

We can simplify this to

\(\displaystyle f'(x)=\lim_{h\rightarrow0}\frac{3x^2h+3xh^2+h^3}{h}\)

Now factor out an h to get

\(\displaystyle f'(x)=\lim_{h\rightarrow0}\frac{h(3x^2+3xh+h^2)}{h}\)

We can simplify and then evaluate the limit.

 

\(\displaystyle f'(x)=\lim_{h\rightarrow0} 3x^2+3xh+h^2\)

\(\displaystyle \\ \\ =3x^2+3x(0)+0^2 \\ =3x^2\)

Example Question #4 : Derivatives

Use the definition of the derivative to solve for \(\displaystyle f'(x)\).

\(\displaystyle f(x)=10x^2\)

Possible Answers:

\(\displaystyle x\)

\(\displaystyle 2x\)

\(\displaystyle 0\)

\(\displaystyle 20x^2\)

\(\displaystyle 20x\)

Correct answer:

\(\displaystyle 20x\)

Explanation:

In order to find \(\displaystyle f'(x)\), we need to remember how to find \(\displaystyle f'(x)\) by using the definition of derivative.

Definition of Derivative:

\(\displaystyle f'(x)=\lim_{h\rightarrow0} \frac{f(x+h)-f(x)}{h}\)

Now lets apply this to our problem.

 

\(\displaystyle f'(x)=\lim_{h\rightarrow0}\frac{10(x+h)^2-10x^2}{h}\)

Now lets expand the numerator.

 \(\displaystyle f'(x)=\lim_{h\rightarrow0}\frac{10x^2+20xh+10h^2-10x^2}{h}\)

We can simplify this to

\(\displaystyle f'(x)=\lim_{h\rightarrow0}\frac{20xh+10h^2}{h}\)

Now factor out an h to get

\(\displaystyle f'(x)=\lim_{h\rightarrow0}\frac{h(20x+10h)}{h}\)

We can simplify and then evaluate the limit.

 

\(\displaystyle f'(x)=\lim_{h\rightarrow0} 20x+10h\)

\(\displaystyle \\ \\ =20x+10(0) \\ =20x\)

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