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AP Calculus BC : Inflection Points

Study concepts, example questions & explanations for AP Calculus BC

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Example Questions

Example Question #25 : Derivatives

$\begin{align*}&\text{Calculate any and all points of inflection for: }\\&f(x)=2x^{3} - 3x^{2} - 2x^{4} - 8x^{5} - 1\end{align*}$

Possible Answers:

-1.424

-0.475

$\text{There are no points of inflection.}$

-3.322

Correct answer:

-0.475

Explanation:

$\begin{align*}&\text{A point of inflection of a function is where its concavity changes}\\&\text{sign. To find the concavity of a function, we'll wish to first}\\&\text{find its second derivative, so let's do it in steps. For the}\\&\text{function we're given, our first derivative is :}\\&6x^{2} - 6x - 8x^{3} - 40x^{4}\\&\text{And our second is:}\\&12x - 24x^{2} - 160x^{3} - 6\\&\text{Now, if a point of inflection exists, it's where this function equals zero with real roots.}\\&\text{Solving for x yields:}\\&x=-0.475,0.162-0.230i,0.162+0.230i\\&\text{We find one real unique root at the point }x=-0.475\\&\text{Now to determine if a point is one of inflection, plug in values on either side of it,}\\&\text{and see if the sign changes. Say }\pm0.005\text{ its value.}\\&\text{With this check we find that at this point the concavity of the function changes.}\end{align*}$

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Example Question #26 : Derivatives

$\begin{align*}&\text{Calculate any and all points of inflection for: }\\&f(x)=3x + 8x^{2} - 3x^{3} - 10x^{4} - 5\end{align*}$

Possible Answers:

-3.134,2.084

-0.448,0.298

$\text{There are no points of inflection.}$

-0.896,1.191

Correct answer:

-0.448,0.298

Explanation:

$\begin{align*}&\text{A point of inflection of a function is where its concavity changes}\\&\text{sign. To find the concavity of a function, we'll wish to first}\\&\text{find its second derivative, so let's do it in steps. For the}\\&\text{function we're given, our first derivative is :}\\&16x - 9x^{2} - 40x^{3} + 3\\&\text{And our second is:}\\&16 - 120x^{2} - 18x\\&\text{Now, if a point of inflection exists, it's where this function equals zero with real roots.}\\&\text{Solving for x yields:}\\&x=-0.448,0.298\\&\text{We find real unique roots at the points }x=-0.448,0.298\\&\text{Now to determine if a point is one of inflection, plug in values on either side of it,}\\&\text{and see if the sign changes. Say }\pm0.005\text{ its value.}\\&\text{With this check we find that at every point the concavity of the function changes.}\end{align*}$

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Example Question #27 : Derivatives

$\begin{align*}&\text{Calculate any and all points of inflection for: }\\&f(x)=\frac{1}{2^{(3x)}}- x - 10x^{2} + x^{3} - 10x^{4} + 6x^{5} + 3\end{align*}$

Possible Answers:

2.182

$\text{There are no points of inflection.}$

7.637

1.091

Correct answer:

1.091

Explanation:

$\begin{align*}&\text{A point of inflection of a function is where its concavity changes}\\&\text{sign. To find the concavity of a function, we'll wish to first}\\&\text{find its second derivative, so let's do it in steps. For the}\\&\text{function we're given, our first derivative is :}\\&3x^{2} -\frac{ (3ln(2))}{2^{(3x)}}- 20x - 40x^{3} + 30x^{4} - 1\\&\text{And our second is:}\\&6x +\frac{ (9ln(2)^{2})}{2^{(3x)}}- 120x^{2} + 120x^{3} - 20\\&\text{For a reminder of derivative rules:}\\&d[b^{ax}]=ab^{ax}ln(b)dx\\&\text{Now, if a point of inflection exists, it's where this function equals zero with real roots.}\\&\text{Solving for x yields:}\\&x=1.091\\&\text{We find one real unique root at the point }x=1.091\\&\text{Now to determine if a point is one of inflection, plug in values on either side of it,}\\&\text{and see if the sign changes. Say }\pm0.005\text{ its value.}\\&\text{With this check we find that at this point the concavity of the function changes.}\end{align*}$

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Example Question #21 : Second Derivatives

$\begin{align*}&\text{Find any points of inflection for the function:}\\&3^{(3x)} - 4x + 10x^{3} + 10x^{4} + 2x^{5}\end{align*}$

Possible Answers:

-0.154

-0.026

$\text{There are no points of inflection.}$

-0.077

Correct answer:

-0.154

Explanation:

$\begin{align*}&\text{A point of inflection of a function is where its concavity changes}\\&\text{sign. To find the concavity of a function, we'll wish to first}\\&\text{find its second derivative, so let's do it in steps. For the}\\&\text{function we're given, our first derivative is :}\\&3\cdot 3^{(3x)}ln(3) + 30x^{2} + 40x^{3} + 10x^{4} - 4\\&\text{And our second is:}\\&60x + 9\cdot 3^{(3x)}ln(3)^{2} + 120x^{2} + 40x^{3}\\&\text{For a reminder of derivative rules:}\\&d[b^{ax}]=ab^{ax}ln(b)dx\\&\text{Now, if a point of inflection exists, it's where this function equals zero with real roots.}\\&\text{Solving for x yields:}\\&x=-0.154\\&\text{We find one real unique root at the point }x=-0.154\\&\text{Now to determine if a point is one of inflection, plug in values on either side of it,}\\&\text{and see if the sign changes. Say }\pm0.005\text{ its value.}\\&\text{With this check we find that at this point the concavity of the function changes.}\end{align*}$

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Example Question #21 : Second Derivatives

$\begin{align*}&\text{Find any points of inflection for the function:}\\&e^{(2x)} - 8x + 8x^{2} + 5x^{3} - 2x^{4} - 7x^{5} - 2\end{align*}$

Possible Answers:

0.330

$\text{There are no points of inflection.}$

0.132

0.660

Correct answer:

0.660

Explanation:

$\begin{align*}&\text{A point of inflection of a function is where its concavity changes}\\&\text{sign. To find the concavity of a function, we'll wish to first}\\&\text{find its second derivative, so let's do it in steps. For the}\\&\text{function we're given, our first derivative is :}\\&16x + 2e^{(2x)} + 15x^{2} - 8x^{3} - 35x^{4} - 8\\&\text{And our second is:}\\&30x + 4e^{(2x)} - 24x^{2} - 140x^{3} + 16\\&\text{For a reminder of derivative rules:}\\&d[e^{ax}]=ae^{ax}dx\\&\text{Now, if a point of inflection exists, it's where this function equals zero with real roots.}\\&\text{Solving for x with a calculator yields:}\\&x=0.660\\&\text{We find one real unique root at the point }x=0.660\\&\text{Now to determine if a point is one of inflection, plug in values on either side of it,}\\&\text{and see if the sign changes. Say }\pm0.005\text{ its value.}\\&\text{With this check we find that at this point the concavity of the function changes.}\end{align*}$

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Example Question #21 : Inflection Points

$\begin{align*}&\text{Find any points of inflection for the function:}\\&4x + ln(5x) - 6x^{2} - 10x^{3} + 6x^{4} - 5\end{align*}$

Possible Answers:

-1.462,5.058

-0.292,1.012

$\text{There are no points of inflection.}$

-0.877,0.506

Correct answer:

-0.292,1.012

Explanation:

$\begin{align*}\\&d[ln(ax)]=\frac{dx}{x}\\&d[x^a]=ax^{a-1}dx\\&\text{Now, if a point of inflection exists, it's where this function equals zero with real roots.}\\&\text{Solving for x with a calculator yields:}\\&x=-0.292,1.012,0.057-0.209i,0.057+0.209i\\&\text{We find real unique roots at the points }x=-0.292,1.012\\&\text{Now to determine if a point is one of inflection, plug in values on either side of it,}\\&\text{and see if the sign changes. Say }\pm0.005\text{ its value.}\\&\text{With this check we find that at every point the concavity of the function changes.}\end{align*}$

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Example Question #31 : Second Derivatives

$\begin{align*}&\text{Calculate any and all points of inflection for: }\\&f(x)=ln(5x) + 8x^{2} - 10x^{3} + 5x^{4} + 3\end{align*}$

Possible Answers:

-0.185,0.651

-1.111,4.555

-0.062,1.301

$\text{There are no points of inflection.}$

Correct answer:

-0.185,0.651

Explanation:

$\begin{align*}&\text{A point of inflection of a function is where its concavity changes}\\&\text{sign. To find the concavity of a function, we'll wish to first}\\&\text{find its second derivative. For the}\\&\text{function we're given, our first derivative is :}\\&16x +\frac{ 1}{x}- 30x^{2} + 20x^{3}\\&\text{And our second is:}\\&60x^{2} -\frac{ 1}{x^{2}}- 60x + 16\\&\text{For a reminder of derivative rules:}\\&d[ln(ax)]=\frac{dx}{x}\\&d[x^a]=ax^{a-1}dx\\&\text{Now, if a point of inflection exists, it's where this function equals zero with real roots.}\\&\text{Solving for x with a calculator yields:}\\&x=-0.185,0.651,0.267-0.259i,0.267+0.259i\\&\text{We find real unique roots at the points }x=-0.185,0.651\\&\text{Now to determine if a point is one of inflection, plug in values on either side of it,}\\&\text{and see if the sign changes. Say }\pm0.005\text{ its value.}\\&\text{With this check we find that at every point the concavity of the function changes.}\end{align*}$

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Example Question #32 : Second Derivatives

$\begin{align*}&\text{Calculate any and all points of inflection for: }\\&f(x)=2^{(6x)} - 6x - 5x^{2} + 3x^{3} + 9\end{align*}$

Possible Answers:

$\text{There are no points of inflection.}$

-0.094

-0.024

-0.013

Correct answer:

-0.094

Explanation:

$\begin{align*}&\text{A point of inflection of a function is where its concavity changes}\\&\text{sign. To find the concavity of a function, we'll wish to first}\\&\text{find its second derivative. For the function we're given, our}\\&\text{first derivative is :}\\&6\cdot 2^{(6x)}ln(2) - 10x + 9x^{2} - 6\\&\text{And our second is:}\\&18x + 36\cdot 2^{(6x)}ln(2)^{2} - 10\\&\text{For a reminder of derivative rules:}\\&d[b^{ax}]=ab^{ax}ln(b)dx\\&\text{Now, if a point of inflection exists, it's where this function equals zero with real roots.}\\&\text{Solving for x with a calculator yields:}\\&x=-0.094\\&\text{We find one real unique root at the point }x=-0.094\\&\text{Now to determine if a point is one of inflection, plug in values on either side of it,}\\&\text{and see if the sign changes. Say }\pm0.005\text{ its value.}\\&\text{With this check we find that at this point the concavity of the function changes.}\end{align*}$

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Example Question #31 : Derivatives

$\begin{align*}&\text{Calculate any and all points of inflection for: }\\&f(x)=10x + 2^{(6x)} + 2x^{2} - 2x^{3}\end{align*}$

Possible Answers:

$\text{There are no points of inflection.}$

-0.160

-0.040

-0.018

Correct answer:

$\text{There are no points of inflection.}$

Explanation:

$\begin{align*}&\text{A point of inflection of a function is where its concavity changes}\\&\text{sign. To find the concavity of a function, we'll wish to first}\\&\text{find its second derivative. For the function we're given, our}\\&\text{first derivative is :}\\&4x + 6\cdot 2^{(6x)}ln(2) - 6x^{2} + 10\\&\text{And our second is:}\\&36\cdot 2^{(6x)}ln(2)^{2} - 12x + 4\\&\text{For a reminder of derivative rules:}\\&d[b^{ax}]=ab^{ax}ln(b)dx\\&\text{Now, if a point of inflection exists, it's where this function equals zero with real roots.}\\&\text{Solving for x with a calculator yields:}\\&x=-0.160-0.553i\\&\text{Since we find no real roots, this function has no points of inflection.}\\&\text{In this case, the function is always concave up.}\\&\end{align*}$

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Example Question #21 : Inflection Points

$\begin{align*}&\text{Find any points of inflection for the function:}\\&2^{(3x)} - 6x + 2x^{2} - 7x^{3} - 9x^{5} + 1\end{align*}$

Possible Answers:

1.070

0.642

$\text{There are no points of inflection.}$

0.214

Correct answer:

0.214

Explanation:

$\begin{align*}&\text{A point of inflection of a function is where its concavity changes}\\&\text{sign. To find the concavity of a function, we'll wish to first}\\&\text{find its second derivative. For the function we're given, our}\\&\text{first derivative is :}\\&4x + 3\cdot 2^{(3x)}ln(2) - 21x^{2} - 45x^{4} - 6\\&\text{And our second is:}\\&9\cdot 2^{(3x)}ln(2)^{2} - 42x - 180x^{3} + 4\\&\text{For a reminder of derivative rules:}\\&d[b^{ax}]=ab^{ax}ln(b)dx\\&\text{Now, if a point of inflection exists, it's where this function equals zero with real roots.}\\&\text{Solving for x with a calculator yields:}\\&x=0.214\\&\text{We find one real unique root at the point }x=0.214\\&\text{Now to determine if a point is one of inflection, plug in values on either side of it,}\\&\text{and see if the sign changes. Say }\pm0.005\text{ its value.}\\&\text{With this check we find that at this point the concavity of the function changes.}\end{align*}$

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AP Calculus BC : Inflection Points

Study concepts, example questions & explanations for AP Calculus BC

All AP Calculus BC Resources

Example Questions

Example Question #25 : Derivatives

Example Question #26 : Derivatives

Example Question #27 : Derivatives

Example Question #21 : Second Derivatives

Example Question #21 : Second Derivatives

Example Question #21 : Inflection Points

Example Question #31 : Second Derivatives

Example Question #32 : Second Derivatives

Example Question #31 : Derivatives

Example Question #21 : Inflection Points

All AP Calculus BC Resources

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