Last year I created a discussion page asking students to create short video lectures per below. We won't ask you to create videos this semester, but students did much much better than usual on chapter 6 exam problems last year. Thus I am copying some of the discussion to this page for an introduction/review of LaPlace transforms. Please ignore comments regarding videos.

In chapter 6 we will use the LaPlace transform to turn a linear DE into an algebra problem using linearity and a table of LaPlace transforms.

For example to solve

$y'' + 3y' + 4y = 0, ~~y(0) = 5, ~~y'(0) = 6$

1.) Take the LaPlace Transform of both sides of the DE equation:

${\cal L}(y'' + 3y' + 4y) = {\cal L}(0)$

2.)  Use the fact that the LaPlace Transform is linear:

${\cal L}(y'') + 3{\cal L}(y') + 4{\cal L}(y) = 0$

3.)  Use formula to change this equation into an algebraic equation:

$[s^2{\cal L}(y) - sy(0) - y'(0)] + 3[s{\cal L}(y) - y(0)] + 4{\cal L}(y) = 0$

3.5)  Substitute in the initial values:

$s^2{\cal L}(y) - 5s - 6 + 3[s{\cal L}(y) - 5] + 4{\cal L}(y) = 0$

4.)  Solve the algebraic equation for ${\cal L}(y)$:

$s^2{\cal L}(y) - 5s - 6 + 3s{\cal L}(y) - 15 + 4{\cal L}(y) = 0\\ ~\\ [s^2 + 3s + 4]{\cal L}(y) = 5s + 21\\ ~\\ {\cal L}(y) = {5s + 21 \over s^2 + 3s + 4}\\ ~\\ \text{Some algebra implies }~ {\cal L}(y) = {5s + 21 \over s^2 + 3s + 4}$

5.)  Solve for $y$  by taking the inverse LaPlace transform of both sides:

$y = {\cal L}^{-1}({\cal L}(y)) = {\cal L}^{-1}\left( {5s + 21 \over s^2 + 3s + 4}\right)$

Once we simplify the RHS, we have solved our DE for by just using algebra and a table of formulas.  Unfortunately simplifying the RHS in step 5 will take the most work, involving a bunch of algebra plus using the LaPlace transform table.  The algebra needed for taking the inverse LaPlace transform will be reviewed in the student created videos below.  This algebra for taking the inverse LaPlace transform is also summarized on page 2 of the LaPlace transform table.  If you wish to see the rest of this problem, the full solution is available by clicking here .  But you can also wait until we cover this in class.  But to prepare for this, please see the discussion below and the following:

Take a look at the LaPlace transform table.  The table we will use is just 1 page, but there are longer versions on the web (but we will just need the formulas in this 1 page table).  To keep track of where we are in the problem, we will use the variable $t$  before we take the LaPlace transform and the variable $s$  after we take the LaPlace transform.

This technique applies to any linear differential equation of any order, but for simplicity, we will start with 2nd order: $y'' + 3y' + 4y = g(t)$

Thus when we take the LaPlace transform of both sides, we will need to be able to calculate ${\cal L}(g(t))$.  To do this, we  look at column 1 in the  LaPlace transform table.  For example if $g(t) = e^{at}$, then  ${\cal L}(g(t)) ={\cal L}(e^{at}) = \frac{1}{s-a}$.  For   $a = 2, ~~~{\cal L}(e^{2t}) = \frac{1}{s-2}$.

Note per above we used the variable $t$  before we take the LaPlace transform and the variable after we take the LaPlace transform.

We will first focus on taking the LaPlace transform before we look at the more involved algebra when taking the inverse LaPlace transform.

We will first do a few examples using linearity and the LaPlace transform table to calculate the LaPlace transform of several functions. To use the LaPlace transform table to calculate ${\cal L}(g(t))$, we look at the first column. Note that after taking the LaPlace transform, we get a function of $s$.

The examples below come from Paul's online notes.

SV LaPlace Video 1:

Use linearity to calculate the LaPlace transform of $f\left( t \right) = 6{{\bf{e}}^{ - 5t}} + {{\bf{e}}^{3t}} + 5{t^3} - 9$.

Include the linearity step:

Also show the formulas that you use.

SV LaPlace Video 2:

Use linearity to calculate the LaPlace transform of g(t)=4cos(4t)9sin(4t)+2cos(10t)

Include the linearity step.  Also show the formulas that you use.

Before looking at inverse transform examples, we will first review some algebra.

Video 5: Ch 6 Complete Square

Review the algebraic technique completing the square.

Example 1:   $s^2 + 3s + 4$

$$s^2 + 3s + \underline{\hskip 0.2in} - \underline{\hskip 0.2in} + 4 = (s + \underline{\hskip0.2in})^2- \underline{\hskip 0.2in} + 4$$ Hence $s^2 + 3s + 4 = \left(s + {3 \over 2}\right)^2 + {7 \over 4}$

Note to complete the square, you add "0" (in this example, "0" takes the form $\frac{9}{4} - \frac{9}{4}$).  When you do algebra, you need to preserve equality.  Thus you can add "0".

Example 2:   Complete the square: $5s^2 + 20s + 15$

Hint: Factor out the 5 and be very careful to preserve equality as you modify the above polynomial in order to complete the square.

## NOTE: You must be able to do partial fractions. For a review of partial fractions, please see Partial fractions examples.

We will now focus on calculating the inverse LaPlace transform (which means going from column 2 with   to column 1 to get .

We will also use the fact that the inverse LaPlace transform is linear plus lots of algebra in order to make your look like (a linear  combination of) something(s) in column 2.

See page 2 of table of LaPlace transforms to see a summary of the main algebraic techniques.  Note we usually focus on the denomator first to determine which formula(s) to use.

Video 10: Ch 6 Inverse LaPlace Transform 1

Calculate the inverse LaPlace transform of   per below:

To calculate the inverse LaPlace transform of , you look at the second column and look for the formula that most closely matches it.  In this case that would be formula 3:

Comparing the denominators of   and , we can see that   and thus .

Letting in formula 3:

We have , but to use the formula, we need .

Fortunately, we are all powerful when it comes to math.  If we want something, we can make it happen, often by multiplying by 1 or adding 0 (so that we don't change the problem).  In this case, we will multiply by $\frac{6}{6} = 1$.

.

Thus

It might sometime feel like you are not all powerful when it comes to math.  However, you really are.  Even if you make lots of algebraic typos (which your professors also do), you are still better than a computer.  It takes a lot of work to program a computer to detect patterns and even then computers are not as good as us in most cases (even IF we make more errors).

See Pauls' online notes Section 4-3 : Inverse Laplace Transforms for practice problems including some of the examples below.

Video 16: Ch 6 Inverse LaPlace Transform 7

Calculate the inverse LaPlace transform of

When calculating the inverse LaPlace transform, look at the denominator.  If it factors over the reals, you will need to use partial fractions.  But in this example, the quadratic polynomial has complex roots and thus does not factor over the reals.  Thus to make it look like the formulas in column 2, you will need to complete the square.

In your video, include the linearity step.  Also show the formulas that you use.  This example is from Pauls' online notes Section 4-3 : Inverse Laplace Transforms. Click on  "Show Answers" for helpful information.

Video 18: Ch 6 Inverse LaPlace Transform 9

Calculate the inverse LaPlace transform of

The denominator in this example factors over the reals.  Thus to make it look like the formulas in column 2, you will need use partial fractions.

In your video, include the linearity step.  Also show the formulas that you use.  This example is from Pauls' online notes Section 4-3 : Inverse Laplace Transforms. Click on  "Show Answers" for helpful information.

We now will use the LaPlace transform to solve an initial value problems.

We will break this into many steps.

To use the LaPlace transform, we need our initial values to be at

Sidenote:  FYI if you are given initial values for a nonzero ,  you can always translate the problem to one where   and translate back.  But all our IVP in ch 6 will have

Video 19: Ch 6 Formula 18

We first need the formula for   where   is the nth derivative of .  This is formula 18, though this formula uses   instead of :

Formula 18:

Since , we have that and the initial values are .

Recall   since .

Thus replacing   with , Formula 18 becomes

Formula 18':

Note that if we take the LaPlace transform of the    =  nth derivative of , we get a degree   polynomial.

Thus

Note the pattern.  Lets now fill in the first blank for each of these.  Note that the coefficient of   in Formula 18 is .

Thus if we fill in just the first blank in each of the above, we get

The remaining blanks are filled in with initial values, starting with y(0).  As the degree of s decreases, the derivative of y (evaluated at 0) increases.  Note we always end with subtracting a constant term as we run out of initial values at the same time we run out of 's.

Thus our formulas become:

Similarly we can calculate the LaPlace transform of any derivative of .

Video 20: Ch 6 Solve

Part 1

Note this is the example in the first post.  We wish to solve for .  It is fairly quick to solve for in terms of an inverse LaPlace transform, but calculating the inverse LaPlace transform will take a few steps.

In this video solve for in terms of an inverse LaPlace transform, but do not calculate the inverse LaPlace transform.  We will save that part of later videos.

1.) Take the LaPlace Transform of both sides of the DE equation:

2.)  Use the fact that the LaPlace Transform is linear:

Note that 0 does not appear in the LaPlace transform table, but for any linear function . Thus we get 0 on the RHS after taking the LaPlace transform of 0.

3.)  Use formula 18 to change this equation into an algebraic equation:

3.5)  Substitute in the initial values:

4.)  Solve the algebraic equation for:

5.)  Solve for   by taking the inverse LaPlace transform of both sides:

We have now solved the differential equation for , but we now want to simplify the RHS by calculating the inverse LaPlace transform.  That will be a lot of work.  Thus we will save that for the next videos.

Video 21: Ch 6 Solve

Part 2:  Calculate

In part 1, we solved the following IVP for    in terms of an inverse LaPlace transform.

Now that we know that

we now want to simplify the RHS by calculating the inverse LaPlace transform.  Per the previous videos, we first see if the denominator factors over the reals.  It does not, so we need to complete the square.  Lots more algebra will then be needed to make it look like a linear combination of 2 formulas in the LaPlace transform table.  Note this algebra involves adding a form of 0 and multiplying by a form of 1.  To see the solution worked out, click here

Video 22: Ch 6 Solve

Part 1 In this video solve for   in terms of an inverse LaPlace transform, but do not calculate the inverse LaPlace transform.  We will save that part for the next video.

Note this is example 1 from https://tutorial.math.lamar.edu/Classes/DE/IVPWithLaplace.aspxy

Some notation:  Many books use .  You are welcome to use either.  Since we are solving for , I like   so that I can see the variable I am solving for. is nice to remind one that we have taken the LaPlace transform and thus is a function of .  Recall that to remind us which column we should use, we use the variable   for before we take the LaPlace transform and the variable for after we have taken the LaPlace transform.

Video 23: Ch 6 Solve

Part 2  Finish solving for

In part 1, we solved the following IVP

for in terms of an inverse LaPlace transform. Now that we know that

we now want to simplify the RHS by calculating the inverse LaPlace transform.  Note the first step is to add the two terms (after getting common denominator) as it will be less work to calculate the inverse LaPlace transform of the sum rather than doing 2 separate inverse LaPlace transforms.  Whether one should combine terms or not depends on the problem, but combining is often less work.

Note this is example 1 from https://tutorial.math.lamar.edu/Classes/DE/IVPWithLaplace.aspxy