Precalculus : Derivatives

Study concepts, example questions & explanations for Precalculus

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Example Questions

Example Question #5 : Maximum And Minimum Problems

Find the coordinates of the relative maximum point of the function 

\displaystyle y=\frac{3}{4}x^4+3x^3-15x^2+8.

Possible Answers:

\displaystyle x=0

\displaystyle (-5,-273.25),(0,8),(2,-16)

\displaystyle (0,8)

\displaystyle (-5,-273.25),(2,-16)

Correct answer:

\displaystyle (0,8)

Explanation:

Let's find the first derivative to locate the relative maxima and minima.

\displaystyle y=\frac{3}{4}x^4+3x^3-15x^2+8

\displaystyle y'=4*\frac{3}{4}x^3+9x^2-30x+0

\displaystyle y'=3x^3+9x^2-30x

Now we set it equal to zero to find the x values of these critical points.

\displaystyle 0=3x^3+9x^2-30x

\displaystyle 0=3x(x^2+3x-10)

\displaystyle 0=3x(x-2)(x+5)

So the equation is 0 where x is -2, 0, or 5. Now let's find the second derivative so that we know which of these locations are maxima and which are minima.

\displaystyle y'=3x^3+9x^2-30x

\displaystyle y''=9x^2+18x-30

\displaystyle y''(-5)=9(-5)^2+18(-5)-30=105>0

So the function has a relative maximum at x=-5.

\displaystyle y''(0)=9(0)^2+18(0)-30=-30< 0

So the function has a relative minimum at x=0.

\displaystyle y''(2)=9(2)^2+18(2)-30=42>0

So the function has a relative maximum at x=2.

Thus there is only one relative minimum in this function, and it occurs at x=0. We need to plug this into the original function to find the y-coordinate of the point.

\displaystyle y=\frac{3}{4}x^4+3x^3-15x^2+8

\displaystyle y=\frac{3}{4}(0)^4+3(0)^3-15(0)^2+8

\displaystyle y=0+0-0+8

\displaystyle y=8

So our point is (0,8).

Example Question #1 : Maximum And Minimum Problems

Find the \displaystyle x-coordinates of the possible locations of the relative maxima and minimia of 

\displaystyle f(x)=\frac{1}{2}x^4-x^2+3.

Possible Answers:

\displaystyle x=0,\pm\sqrt2

There are no critical points.

\displaystyle x=\pm\sqrt\frac{1}{3}

\displaystyle x=0,\pm 1

Correct answer:

\displaystyle x=0,\pm 1

Explanation:

We need to find out where the first derivative is equal to zero to find the locations of the possible maxima and minima.

\displaystyle f(x)=\frac{1}{2}x^4-x^2+3

\displaystyle f'(x)=\frac{4}{2}x^3-2x+0

\displaystyle f'(x)=2x^3-2x

\displaystyle f'(x)=2x(x^2-1)

\displaystyle f'(x)=2x(x+1)(x-1)

\displaystyle 0=2x(x+1)(x-1)

\displaystyle x=0,1,-1 or \displaystyle x=0,\pm1.

Example Question #1 : Maximum And Minimum Problems

Without solving the problem, determine whether the function \displaystyle y=6x^2-2x+9 will have relative maxima or minima and how many of each.

Possible Answers:

No critical points

One relative maximum

One minimum and one maximum

One relative minimum

Correct answer:

One relative minimum

Explanation:

We see that the function is a quadratic, so its graph will be a parabola. Thus, we know it will have only one relative max or min. Since the leading coefficient of \displaystyle y=6x^2-2x+9 is a positive 6, we also know that the parabola opens upward. Hence, the relative extremum is a minimum.

Example Question #61 : Introductory Calculus

Find the second derivative of the following function:

\displaystyle f(x)=2x^5-x^4+\frac{3}{4}x^3+\frac{5}{2}x^2-11x

Possible Answers:

\displaystyle f''(x)=20x^3+4x^2-\frac{9}{4}x-5

\displaystyle f''(x)=10x^4-4x^3+\frac{9}{4}x^2+5x-11

\displaystyle f''(x)=2x^3-x^2+\frac{3}{4}x+\frac{5}{2}

\displaystyle f''(x)=80x^3-6x^2+\frac{9}{4}x+10

\displaystyle f''(x)=40x^3-12x^2+\frac{9}{2}x+5

Correct answer:

\displaystyle f''(x)=40x^3-12x^2+\frac{9}{2}x+5

Explanation:

In order to take any order derivative of a polynomial, all we need to know is how to apply the power rule to a simple term with an exponent:

\displaystyle \frac{d}{dx}(Ax^n)=nAx^{n-1}

The formula above tells us that to take the derivative of a term with coefficient \displaystyle A and exponent \displaystyle n, we simply multiply the term by \displaystyle n and subtract 1 from \displaystyle n in the exponent. With this in mind, we'll take the first derivative of the given function, and then apply the power rule to each term once again to find the second derivative of the given function:

\displaystyle f(x)=2x^5-x^4+\frac{3}{4}x^3+\frac{5}{2}x^2-11x

\displaystyle f'(x)=10x^4-4x^3+\frac{9}{4}x^2+5x-11

Now if we take the derivative of the first derivative, we'll get the second derivative of our function:

\displaystyle f'(x)=10x^4-4x^3+\frac{9}{4}x^2+5x-11

\displaystyle f''(x)=40x^3-12x^2+\frac{9}{2}x+5

Example Question #62 : Introductory Calculus

Find the second derivative of the function \displaystyle f(x)=7x^{3}.

Possible Answers:

\displaystyle 21x^{2}

\displaystyle 21x^{3}

\displaystyle 10x

\displaystyle 42x

\displaystyle 42x^{2}

Correct answer:

\displaystyle 42x

Explanation:

To take the derivative of a function, we'll need to apply the power rule to a term with a coefficient \displaystyle A and an exponent \displaystyle n:

\displaystyle \frac{d}{dx}(Ax^{n})=nAx^{n-1} 

Applying this rule to each term in the function, we start by taking the first derivative:

\displaystyle f(x)=7x^{3}

\displaystyle f'(x)=3(7x^{3-1})

\displaystyle f'(x)=21x^{2}

Taking the second derivative:

\displaystyle f''(x)=2(21x^{2-1})

\displaystyle f''(x)=42x

 

Example Question #2 : Find The Second Derivative Of A Function

Find the second derivative of the function

 \displaystyle f(x)=2x^{4}-12x^{3}+x^{2}-8x+5

Possible Answers:

\displaystyle f''(x)=24x^{3}-123x^{2}+2x-8

\displaystyle f''(x)=8x^{3}-36x^{2}+2x-8x+5

\displaystyle f''(x)=8x^{3}-15x^{2}+2x-8

\displaystyle f''(x)=24x^{2}-72x+2

\displaystyle f''(x)=8x^{3}-36x^{2}+2x-8x

Correct answer:

\displaystyle f''(x)=24x^{2}-72x+2

Explanation:

To take the derivative of a function, we'll need to apply the power rule to a term with a coefficient \displaystyle A and an exponent \displaystyle n:

\displaystyle \frac{d}{dx}(Ax^{n})=nAx^{n-1} 

Applying this rule to each term in the function, we start by taking the first derivative:

\displaystyle f(x)=2x^{4}-12x^{3}+x^{2}-8x+5

\displaystyle f'(x)=4(2x^{4-1})-3(12x^{3-1})+2(x^{2-1})-1(8x^{1-1})+0(5)

\displaystyle f'(x)=8x^{3}-36x^{2}+2x-8

Finally, we take the second derivative:

\displaystyle f'(x)=8x^{3}-36x^{2}+2x-8x

\displaystyle f''(x)=3(8x^{3-1})-2(36x^{2-1})+1(2x^{1-1})-0(8)

\displaystyle f''(x)=24x^{2}-72x+2

Example Question #3 : Find The Second Derivative Of A Function

Find the second derivative of the function \displaystyle f(x)=6x^{2}+14x-2

Possible Answers:

\displaystyle f''(x)=12x+14

\displaystyle f''(x)=12

\displaystyle f''(x)=8x+15

\displaystyle f''(x)=12x

\displaystyle f''(x)=12x+12

Correct answer:

\displaystyle f''(x)=12

Explanation:

To take the derivative of a function, we'll need to apply the power rule to a term with a coefficient \displaystyle A and an exponent \displaystyle n:

\displaystyle \frac{d}{dx}(Ax^{n})=nAx^{n-1} 

Applying this rule to each term in the function, we start by taking the first derivative:

\displaystyle f(x)=6x^{2}+14x-2

\displaystyle f'(x)=2(6x^{2-1})+1(14x^{1-1})-0(2)

\displaystyle f'(x)=12x+14

Then, taking the second derivative of the function:

\displaystyle f'(x)=12x+14

\displaystyle f''(x)=1(12x^{1-1})+0(14)

\displaystyle f''(x)=12

Example Question #21 : Derivatives

What is the second derivative of

\displaystyle y(x)=x

with respect to \displaystyle x

Possible Answers:

\displaystyle x^3

\displaystyle 0

\displaystyle 1

\displaystyle x

\displaystyle x^2

Correct answer:

\displaystyle 0

Explanation:

We first apply Power Rule.

First Derivative \displaystyle x^1:

\displaystyle 1-1=0

So result is

\displaystyle 1x^0

Anything to a power of \displaystyle 0 is \displaystyle 1

First Derivative is

\displaystyle 1

 

Second Derivative \displaystyle 1:

Any derivative of a constant is \displaystyle 0

 

Second Derivative of \displaystyle x with respect to \displaystyle x is

\displaystyle 0

Example Question #1 : Find The Second Derivative Of A Function

Find the second derivative of

\displaystyle y(x)=7x^5

with respect to \displaystyle x

Possible Answers:

\displaystyle 150x^3

\displaystyle 420x^2

\displaystyle 140x^3

\displaystyle x^5

\displaystyle 35x^4

Correct answer:

\displaystyle 140x^3

Explanation:

Use Power Rule to take two derivatives of \displaystyle 7x^5:

First Derivative:

\displaystyle 7\cdot5=35

\displaystyle 5-1=4

So result is:

\displaystyle 35x^4

 

Now we take another derivative:

Second Derivative:

\displaystyle 35\cdot4=140

\displaystyle 4-1=3

So our result is:

\displaystyle 140x^3

Example Question #23 : Derivatives

What is the second derivative of

\displaystyle y(x) = x^2

with respect to \displaystyle x.

Possible Answers:

\displaystyle 0

\displaystyle x

\displaystyle 2x

\displaystyle x^2

\displaystyle 2

Correct answer:

\displaystyle 2

Explanation:

Apply Power Rule twice.

First Derivative of \displaystyle x^2:

\displaystyle 2\cdot1=2

\displaystyle 2-1=1

So our result is

\displaystyle 2x^1

 

Second Derivative of \displaystyle x^2:

\displaystyle 2\cdot1=2

\displaystyle 1-1=0

So our result is

\displaystyle 2x^0

 

So the second derivative of \displaystyle x^2 is

\displaystyle 2

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