Monthly Archives: March 2015

OpenLab #3: Flipping the class – Taylor Series

Hi everyone,

After Spring Break we will be launching into a new and incredibly useful topic – using Taylor Series to solve differential equations.  Of course, using this method requires you to remember something about Taylor Series (uh oh!).  You should have studied them in Calculus, probably Calculus II or III.  Don’t remember? This is your chance to refresh your memory.

Assignment (Due Tuesday, April 14th, BEFORE CLASS):  Watch all of the following videos carefully.  As you are watching, make a note of any questions you have.  When you are done, complete the following exercise:

Exercise:  Find at least the first 10 terms of the Taylor Series for y=e^{2x} at the point x=0.

Finally, post a comment here confirming that you watched the videos, and including the following:

  1. Confirm that you watched the videos (say something like “I watched all the videos.”)
  2. Post ONE of the terms you found while solving the exercise above (for example, you might say “One of the terms is \frac{4x^5}{15}“).  CHALLENGE: Don’t use the same term as anyone else.
  3. Ask a question about Taylor Series, either about one of the videos (tell me which one), or about Taylor Series in general.  If you don’t have a question, tell me something about Taylor Series – what do you like about them, what don’t you like, what’s the big deal, etc…

Here are the videos:

Video 1: Taylor/Maclauren Series intuition

 

Video 2: Taylor Series for Cosine at x=0 (Maclaurin Series)

Video 3 (OPTIONAL): Taylor Series for Sine at x=0  (NOTE: This video is OPTIONAL, but it might help you better understand the next video)

Video 4: Visualizing Taylor Series Approximation

Undetermined Coefficients: What happens when everything cancels?

An excellent question that I received in email today with regards to WeBWorK #9:

Hi professor Reitz, on problem number two for the new homework, when I try to solve for the particular solution, everything on the left side cancels.

This will happen when the expression on the right side of the equation also happens to be one of the solutions to the homogeneous equation.  We deal with it in much the same way we dealt with repeated roots in homogeneous equations: When guessing the particular solution to the nonhomogeneous equation, multiply your guess by t (for example, use y=Cte^{t} instead of y=Ce^{t}.  Here’s an example.

Example: y''-7y'+10y = 10e^{2t}

The general solution to the associated homogeneous equation y''-7y'+10y=0 is:

General solution: y=Ae^{2t}+Be^{5t}

Notice that one of the basic solutions involves e^{2t}, which matches the right hand side of the original equation.  Because of this, we would make the following guess for a particular solution:

Guess: y=Cte^{2t}

Notice that when you take the derivative, you will still end up with a term involving just Ce^{2t} (without the extra “t”), which will allow the left hand side of the equation to equal the 10e^{2t} on the right side.

Let me know if you have any questions (post a comment!),
Prof. Reitz

WeBWorK extension

WeBWorK assignments #6, 7, 8, formerly due tonight, have been extended by two days.  They are now due this Thursday, 3/26, at midnight.

Regards,
Prof. Reitz

Exam 2 Review is posted

UPDATE: I added one problem (#5) to the Exam Review, from the topic exact equations —  you may remember that we covered this in class just before the first exam, but it was not on the first exam.

Hi everyone,

The review sheet for Exam #2 (which will take place on Thursday, April 2nd) is posted on the Handouts page.  Let me know if you have any questions.

Best regards,
Prof. Reitz

WeBWorK 6: What are these “basic solutions” in part c) of the questions??

Hi everyone,

Just a quick note about part c) in problems like these:

Screen Shot 2015-03-20 at 1.12.05 AM

 

The “basic solutions” refer to the two solutions that come from plugging the roots of the characteristic equation into our “best guess” formula y=e^{rt}.

For example, if the two roots are r=3, r=-5, then the basic solutions would be:

e^{3t}, e^{-5t}

Let me know if you have any followup questions  (or questions about anything else in the current WeBWorK assignment).

Regards,
Prof. Reitz

OpenLab #2: Differential Equations Study Guide (part 1 – topic selection)

Hi everyone,

This is the beginning of a multi-part OpenLab assignment focussed on building resources for you and your fellow students.  Our ultimate goal will be to create a study guide for this course, with videos and other resources for each topic.

Assignment (Due next Thursday, 3/26).  Your first assignment is to choose a topic to work on.  Fill in the form below, selecting THREE different topics that you would be interested in researching for our Study Guide (you will be assigned one of your three choices).  Feel free to choose a topic that we have not yet studied (you will be given ample time to work on your portion of the Study Guide after we cover the topic in class).

You must be logged in to the OpenLab to complete this form. Having trouble with your account? Look here for help and resources: https://openlab.citytech.cuny.edu/2015-spring-mat-2680-reitz/?p=39

 

 

Office hours cancelled today (Thursday, 3/19)

Hi everyone,

My office hours are cancelled today (Thursday, 3/19) as I will be observing another class.  I will be available directly after class for brief questions – if you want to meet with me for a longer period please let me know.

Best,
Prof. Reitz

Second order ordinary homogeneous differential equations with constant coefficients

Here are some video resources showing complete examples (from Khan Academy).  I hope they help!

-Prof. Reitz

Example 1 — in which the characteristic equation has two distinct real roots.

 

Example 2 — in which the characteristic equation has one repeated (real) root.

 

Example 3 — in which the characteristic equation has complex roots.

Euler’s Method Exercises

Hi everyone,

Here are some examples of Euler’s Method problems for you to try.  These are for practice only, they will not be collected.
NOTE: The first example is the one we solved in class on Thursday – I’m including it here for your reference.

Questions? Problems? Leave a comment on this post and I’ll respond here.

Example 1.  Given the initial value problem \frac{dy}{dt} = 3-2t-0.5y with y(0)=1, approximate the value of y(1).  Use 5 steps.

Example 2.  Consider y' = y \sin{t} -1.  Suppose y(0)=2.  What is y(3)?  Use a step size of h=0.5.

Example 3. If y'=\frac{x-4}{x-y+1}, use Euler’s method with 4 steps to approximate y(8) for the particular solution satisfying y(3)=3

Example 4.  Given \frac{dy}{dt}=e^\frac{t^2}{y^2+1} - y with y(-0.82)=0.97, estimate the value of y(1.28) using 6 steps.

Solutions to these examples are provided here for your reference – if you notice an error, please let me know by leaving a comment below (or sending an email).

Office Hours rescheduled Thursday 3/11/15

Hi everyone,

I will be observing another class from 9-10am this Thursday 3/11, so my office hours will take place:
12:30pm – 1:30pm, on Thursday 3/11, in N707

Best regards,
Prof. Reitz