# Differential Equations (Type 6)

### AP Questions Type 6: Differential Equations

Differential equations are tested in the free-response section of the AP exams almost every year. The actual solving of the differential equation is usually the main part of the problem accompanied by a related question such as a slope field or a tangent line approximation. BC students may also be asked to approximate using Euler’s Method. Several parts of the BC questions are often suitable for AB students and contribute to the AB sub-score of the BC exam. This topic may also appear in the multiple-choice sections of the exams. What students should be able to do
• Find the general solution of a differential equation using the method of separation of variables (this is the only method tested).
• Find a particular solution using the initial condition to evaluate the constant of integration – initial value problem (IVP).
• Determine the domain restrictions on the solution of a differential equation. See this post for more on the domain of a differential equation.
• Understand that proposed solution of a differential equation is a function (not a number) and if it and its derivative are substituted into the given differential equation the resulting equation is true. This may be part of doing the problem even if solving the differential equation is not required (see 2002 BC 5 – parts a, b and d are suitable for AB)
• Growth-decay problems.
• Draw a slope field by hand.
• Sketch a particular solution on a given slope field.
• Interpret a slope field.
• Multiple-choice: Given a differential equation, identify is slope field.
• Multiple-choice: Given a slope field identify its differential equation.
• Use the given derivative to analyze a function such as finding extreme values
• For BC only: Use Euler’s Method to approximate a solution.
• For BC only: use the method of partial fractions to find the antiderivative after separating the variables.
• For BC only: understand the logistic growth model, its asymptotes, meaning, etc. The exams so far, have never asked students to actually solve a logistic equation IVP
Look at the scoring standards to learn how the solution of the differential equation is scored, and therefore, how students should present their answer. This is usually the one free-response answer with the most points riding on it. Starting in 2016 the scoring has changed slightly. The five points are now distributed this way:
• one point for separating the variables
• one point each for finding the antiderivatives
• one point for including the constant of integration and using the initial condition – that is, for writing “+ C” on the paper with one of the antiderivatives and substituting the initial condition; finding the value of C is included in the “answer point.” (In the older exams one point was earned for writing the +C and another point for using the initial condition.)
• one point for solving for y: the “answer point”, for the correct answer. This point includes all the algebra and arithmetic in the problem including solving for C.
In the past, the domain of the solution was often included on the scoring standard, but unless it was specifically asked for in the question students did not need to include it. However, the CED. lists “EK 3.5A3 Solutions to differential equations may be subject to domain restrictions.” Perhaps this will be asked in the future. For more on domain restrictions with examples see this post. Shorter questions on this concept appear in the multiple-choice sections. As always, look over as many questions of this kind from past exams as you can find. For some previous posts on differential equations see January 5, 2015, and for post on related subjects see November 26, 2012, January 21, 2013, February 16, 2013 The Differential Equation question covers topics in Unit 7 of the CED.
Free-response examples:
• 2019 There was no DE question in the free-response. You may assume the topic was tested in the multiple-choice sections.
• 2017 AB4/BC4,
• 2016 AB 4, BC 4, (different questions)
• 2015 AB4/BC4,
• 2013 BC 5
• and a favorite Good Question 2 and Good Question 2 Continued
• 2021 AB 6, BC 5 (b), (c)
• 2022 AB5 – sketch solution on slope field, tangent line approximation, solve separable equation.
Multiple-choice examples from non-secure exams:
• 2012 AB 23, 25
• 2012 BC: 12, 14, 16, 23

### Previous posts on these topics for both AB and BC include:

Differential Equations  A summary of the terms and techniques of differential equations and the method of separation of variables Domain of a Differential Equation – On domain restrictions. Accumulation and Differential Equations  Slope Fields An Exploration in Differential Equations An exploration illustrating many of the ideas of differential equations. The exploration is here in PDF form and the solution is here. The ideas include: finding the general solution of the differential equation by separating the variables, checking the solution by substitution, using a graphing utility to explore the solutions for all values of the constant of integration, finding the solutions’ horizontal and vertical asymptotes, finding several particular solutions, finding the domains of the particular solutions, finding the extreme value of all solutions in terms of C, finding the second derivative (implicit differentiation), considering concavity, and investigating a special case or two.

### Previous Posts on BC Only Topics

Euler’s Method Euler’s Method for Making Money The Logistic Equation  Logistic Growth – Real and Simulated

Revised 2/20/2021, March 29, May 14, 2022
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# Adapting 2021 BC 5

Eight of nine. We continue our study of the 2021 free-response questions. We will look at ways to adapt, expand, and explore this question to help students better understand it and look at other questions that can be asked based on a similar stem.

## 2021 BC 5

This is a Differential Equation (Type 6) with a Sequence and Series (Type 10) question included. It contains topics from Units 7 and 10 of the current Course and Exam Description (CED). It is not unusual for AP Calculus exam question to include several of the types in my classification and from several of the units from the CED (here units 7 and 10). In addition, the usual solving an initial value problem and a Euler’s Method approximation are included.

The stem for 2021 BC 5 is:

Part (a): Students were asked to write the second-degree Taylor polynomial for the function centered at x = 1 and then use it to approximate f(2). Students should stop after substituting 2 into their polynomial; no arithmetic or simplification is required, and a simplifying mistake will lose a point.

Discussion and ideas for adapting this question:

• Ask students to find an expression for the second derivative (implicit differentiation).
• Verify that $\displaystyle {f}''\left( 1 \right)=4$
• Ask students to find the third-degree polynomial and use it to approximate f(2)

Part (b): Students were required to approximate f(2) using Euler’s method with two steps of equal size.

Discussion and ideas for adapting this question:

• After you solve the equation in part (c), ask students to compare the approximations from parts (a) and (b) with the exact value. Neither approximation is very close to the exact value. Discuss why this is so. Consider the slope of the graph near x = 2.
• Find a more accurate approximation using 3, 4, or more smaller steps. There are graphing calculator programs that will do the arithmetic. Do not hesitate to use them. Students have already shown they know how to do a Euler’s Method approximation; the point is to understand the situation.

Part (c): Finding the solution of the differential equation by separating the variables is expected in this kind of question. The added twist is that the method of integration by parts is necessary to find one of the antiderivatives.

Discussion and ideas for adapting this question:

• Be sure not to skip over removing the absolute value signs. The most efficient way is to realize that at (and near) the initial condition y > 0, so |y| = y. What do you do if y < 0?
• There is not much you can do differently here. One thing is to change the initial condition. Try a negative value such as f(1) = –4.
• As suggested in (b), compare, and discuss the approximations with the exact value.

Next week we will conclude this series of posts with a look at 2021 BC 6.

I would be happy to hear your ideas for other ways to use this question. Please use the reply box below to share your ideas.

# Differential Equations (Type 6)

### AP  Questions Type 6: Differential Equations

Differential equations are tested almost every year. The actual solving of the differential equation is usually the main part of the problem, but it is accompanied by a related question such as a slope field or a tangent line approximation. BC students may also be asked to approximate using Euler’s Method. Large parts of the BC questions are often suitable for AB students and contribute to the AB sub-score of the BC exam. What students should be able to do

• Find the general solution of a differential equation using the method of separation of variables (this is the only method tested).
• Find a particular solution using the initial condition to evaluate the constant of integration – initial value problem (IVP).
• NEW Determine the domain restrictions on the solution of a differential equation. See this post for more on this.
• Understand that proposed solution of a differential equation is a function (not a number) and if it and its derivative are substituted into the given differential equation the resulting equation is true. This may be part of doing the problem even if solving the differential equation is not required (see 2002 BC 5 – parts a, b and d are suitable for AB)
• Growth-decay problems.
• Draw a slope field by hand.
• Sketch a particular solution on a given slope field.
• Interpret a slope field.
• Multiple-choice: Given a differential equation, identify is slope field.
• Multiple-choice: Given a slope field identify its differential equation.
• Use the given derivative to analyze a function such as finding extreme values
• For BC only: Use Euler’s Method to approximate a solution.
• For BC only: use the method of partial fractions to find the antiderivative after separating the variables.
• For BC only: understand the logistic growth model, its asymptotes, meaning, etc. The exams so far, have never asked students to actually solve a logistic equation IVP

Look at the scoring standards to learn how the solution of the differential equation is scored, and therefore, how students should present their answer. This is usually the one free-response answer with the most points riding on it. Starting in 2016 the scoring has changed slightly. The five points are now distributed this way:

• one point for separating the variables
• one point each for finding the antiderivatives
• one point for including the constant of integration and using the initial condition – that is, for writing “+ C” on the paper with one of the antiderivatives and substituting the initial condition; finding the value of C is included in the “answer point.” and
• one point for solving for y: the “answer point”, for the correct answer. This point includes all the algebra and arithmetic in the problem including solving for C..

In the past, the domain of the solution is often included on the scoring standard, but unless it is specifically asked for in the question students do not need to include it. However, the 9 CED. lists “EK 3.5A3 Solutions to differential equations may be subject to domain restrictions.” Perhaps this will be asked in the future. For more on domain restrictions with examples see this post. Shorter questions on this concept appear in the multiple-choice sections. As always, look over as many questions of this kind from past exams as you can find. For some previous posts on differential equations see January 5, 2015 and for post on related subjects see November 26, 2012,  January 21, 2013 February 16, 2013 The Differential Equation question covers topics in Unit 7 of the 2019 CED.

Free-response examples:

• 2019 There was no DE question in the free-response. You may assume the topic was tested in the multiple-choice sections.
• 2017 AB4/BC4,
• 2016 AB 4, BC 4, (different questions)
• 2015 AB4/BC4,
• 2013 BC 5
• and a favorite  Good Question 2 and Good Question 2 Continued

Multiple-choice examples from non-secure exams:

• 2012 AB 23, 25
• 2012 BC: 12, 14, 16, 23

### Previous posts on these topics for both AB and BC include:

Differential Equations  A summary of the terms and techniques of differential equation and the method of separation of variables

Domain of a Differential Equation – On domain restrictions.

Accumulation and Differential Equations

Slope Fields

An Exploration in Differential Equations An exploration illustrating many of the ideas of differential equations. The exploration is here in PDF form and the solution is here. The ideas include: finding the general solution of the differential equation by separating the variables, checking the solution by substitution, using a graphing utility to explore the solutions for all values of the constant of integration, finding the solutions’ horizontal and vertical asymptotes, finding several particular solutions, finding the domains of the particular solutions, finding the extreme value of all solutions in terms of C, finding the second derivative (implicit differentiation), considering concavity, and investigating a special case or two.

### Previous Posts on BC Only Topics

Euler’s Method

Euler’s Method for Making Money

The Logistic Equation

Logistic Growth – Real and Simulated

Revised 2/20/2021

# 2019 CED Unit 7: Differential Equations

Applications (Unit 8) seem to fit more logically after the opening unit on integration (Unit 6). The Course and Exam Description (CED) present differential equations first probably because the previous unit ended with techniques of antidifferentiation. My guess is that many teachers will teach Unit 8: Applications of Integration before Unit 7: Differential Equations. Therefore, for those who want to present unit 8 first, I will post unit 8 next week on December 3, 2019. That way you’ll have both for reference and can choose the order you think will work best for your students.

Unit 7 is an introduction to the initial ideas and easy techniques related to differential equations. (CED – 2019 p. 129 – 142). These topics account for about 6 – 12% of questions on the AB exam and 6 – 9% of the BC questions.

### Topics 7.1 – 7.9

Topic 7.1 Modeling Situations with Differential Equations Relating a function and its derivatives.

Topic 7.2 Verifying Solutions for Differential Equations A proposed solution to a differential equation can be checked by substituting the function and its derivative(s) into the original differential equation. There may be an infinite number of general solutions (solutions with one or more constants).

Topic 7.3 Sketching Slope Fields Slope fields are a graphical representation of a differential equation and provide information about the behavior of the solutions.

Topic 7.4 Reasoning Using Slope Fields

Topic 7.5 Approximating Solutions Using Euler’s method (BC ONLY) A numerical approach to approximating solutions of a differential equation.

Topic 7.6 Finding General Solutions Using Separation of Variable Since this unit is only an introduction to differential equations, the method of separation of variable is the only solution method tested on the AB and BC exams.

Topic 7.7 Finding Particular Solutions Using Initial Conditions and Separation of Variables An initial condition (i.e. a point on the particular solution) allows you to evaluate the constant in the general solution and find the one solution that contains the initial condition. Also, if $\displaystyle \frac{{dy}}{{dx}}=f\left( x \right)$ has the initial condition $\displaystyle \left( {a,F\left( a \right)} \right)$, then the solution is $\displaystyle F\left( x \right)=F\left( a \right)+\int_{a}^{x}{{f\left( x \right)dx}}$. Solution may also be subject to domain restrictions

Topic 7.8 Exponential Models with Differential Equations Applications include linear motion and exponential growth and decay. The growth and decay model is $\displaystyle \frac{{dy}}{{dt}}=ky$ with the initial condition$\displaystyle (0,{{y}_{0}})$ has the solution $\displaystyle y={{y}_{0}}{{e}^{{kt}}}$.

Topic 7.9 Logistic Models with Differential Equations (BC ONLY) The model of logistic growth, $\displaystyle \frac{{dy}}{{dt}}=ky\left( {a-y} \right)$, can be solved by separating the variables and using partial fraction decomposition. This has never been tested (probably because solving requires a large amount of complicated algebra). Students are expected to know how to interpret the properties of the solution directly from the differential equation (asymptotes, carrying capacity, point where changing the fastest, etc.) and discuss what they mean in context without actually solving the equation.

### Timing

The suggested time for Unit 7 is  8 – 9 classes for AB and 9 – 10 for BC of 40 – 50-minute class periods, this includes time for testing etc.

### Previous posts on these topics for both AB and BC include:

Differential Equations A summary of the terms and techniques about differential equations and the method of separation of variables

Domain of a Differential Equation – On domain restrictions.

Accumulation and Differential Equations

Slope Fields

An Exploration in Differential Equations An exploration illustrating many of the ideas of differential equations. The exploration is here in PDF form and the solution is here. The ideas include: finding the general solution of the differential equation by separating the variables, checking the solution by substitution, using a graphing utility to explore the solutions for all values of the constant of integration, finding the solutions’ horizontal and vertical asymptotes, finding several particular solutions, finding the domains of the particular solutions, finding the extreme value of all solutions in terms of C, finding the second derivative (implicit differentiation), considering concavity, and investigating a special case or two.

### Posts on BC Only Topics

Euler’s Method

Euler’s Method for Making Money

The Logistic Equation

Logistic Growth – Real and Simulated

Here are links to the full list of posts discussing the ten units in the 2019 Course and Exam Description.

2019 CED – Unit 1: Limits and Continuity

2019 CED – Unit 2: Differentiation: Definition and Fundamental Properties.

2019 CED – Unit 3: Differentiation: Composite , Implicit, and Inverse Functions

2019 CED – Unit 4 Contextual Applications of the Derivative  Consider teaching Unit 5 before Unit 4

2019 – CED Unit 5 Analytical Applications of Differentiation  Consider teaching Unit 5 before Unit 4

2019 – CED Unit 6 Integration and Accumulation of Change

2019 – CED Unit 7 Differential Equations  Consider teaching after Unit 8

2019 – CED Unit 8 Applications of Integration   Consider teaching after Unit 6, before Unit 7

2019 – CED Unit 9 Parametric Equations, Polar Coordinates, and Vector-Values Functions

2019 CED Unit 10 Infinite Sequences and Series

# Type 6 Questions: Differential Equations

Differential equations are tested every year. The actual solving of the differential equation is usually the main part of the problem, but it is accompanied by a related question such as a slope field or a tangent line approximation. BC students may also be asked to approximate using Euler’s Method. Large parts of the BC questions are often suitable for AB students and contribute to the AB sub-score of the BC exam.

What students should be able to do

• Find the general solution of a differential equation using the method of separation of variables (this is the only method tested).
• Find a particular solution using the initial condition to evaluate the constant of integration – initial value problem (IVP).
• NEW Determine the domain restrictions on the solution of a differential equation. See this post for more on this.
• Understand that proposed solution of a differential equation is a function (not a number) and if it and its derivative are substituted into the given differential equation the resulting equation is true. This may be part of doing the problem even if solving the differential equation is not required (see 2002 BC 5 – parts a, b and d are suitable for AB)
• Growth-decay problems.
• Draw a slope field by hand.
• Sketch a particular solution on a given slope field.
• Interpret a slope field.
• Multiple-choice: Given a differential equation, identify is slope field.
• Multiple-choice: Given a slope field identify its differential equation.
• Use the given derivative to analyze a function such as finding extreme values
• For BC only: Use Euler’s Method to approximate a solution.
• For BC only: use the method of partial fractions to find the antiderivative after separating the variables.
• For BC only: understand the logistic growth model, its asymptotes, meaning, etc. The exams so far, have never asked students to actually solve a logistic equation IVP

Look at the scoring standards to learn how the solution of the differential equation is scored, and therefore, how students should present their answer. This is usually the one free-response answer with the most points riding on it. Starting in 2016 the scoring has changed slightly. The five points are now distributed this way:

• one point for separating the variables
• one point each for finding the antiderivatives
• one point for including the constant of integration and using the initial condition – that is, for writing “+ C” on the paper with one of the antiderivatives and substituting the initial condition; finding the value of C is included in the “answer point.” and
• one point for solving for y: the “answer point”, for the correct answer. This point includes all the algebra and arithmetic in the problem including solving for C..

In the past, the domain of the solution is often included on the scoring standard, but unless it is specifically asked for in the question students do not need to include it. However, the new CED lists “EK 3.5A3 Solutions to differential equations may be subject to domain restrictions.” Perhaps this will be asked in the future. For more on domain restrictions with examples see this post.

Shorter questions on this concept appear in the multiple-choice sections. As always, look over as many questions of this kind from past exams as you can find.

For some previous posts on differential equations see January 5, 2015 and for post on related subjects see November 26, 2012,  January 21, 2013 February 16, 2013

Free-response examples:

Multiple-choice examples from non-secure exams:

• 2012 AB 23, 25
• 2012 BC: 12, 14, 16, 23

Schedule of review postings:

# Posts on Differential Equations – 2

More posts on differential equations

Good Question 2 (2002 BC 5) and A Family of Function (Good Question 2 continued) – one of my all-time favorite AP exam questions. Parts a, c, and d are suitable for AB students. Part b is a Euler’s method question. Part d is an example of a question where the second derivative test is the only approach possible.

Euler’s method for Making Money an application

The Logistic Equation  – a BC only topic

Logistic Growth – Real and Simulated  – a BC only topic

Logarithms  Defining logarithms with a differential equation

# Posts on Differential Equations – 1

Next in line are differential equations. Here are links to some past posts on differential equations

Differential Equations Outline of basic ideas for AB and BC calculus

Slope Fields

Euler’s Method – a BC only topic

Domain of a Differential Equation mentioned on the new Course and Exam Description

Good Question 6  2000 AB 4

Additional post on differential equations next week.