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### 1.3Separable equations

Note: 1 lecture, §1.4 in [EP], §2.2 in [BD]

When a diﬀerential equation is of the form , we can just integrate: . Unfortunately this method no longer works for the general form of the equation . Integrating both sides yields

Notice the dependence on in the integral.

#### 1.3.1Separable equations

Let us suppose that the equation is separable. That is, let us consider

for some functions and . Let us write the equation in the Leibniz notation

Then we rewrite the equation as

Now both sides look like something we can integrate. We obtain

If we can ﬁnd closed form expressions for these two integrals, we can, perhaps, solve for .

Example 1.3.1: Take the equation

First note that is a solution, so assume from now on. Write the equation as , then

We compute the antiderivatives to get

Or

where is some constant. Because is a solution and because of the absolute value we actually can write:

for any number (including zero or negative).

We check:

Yay!

We should be a little bit more careful with this method. You may be worried that we were integrating in two diﬀerent variables. We seemed to be doing a diﬀerent operation to each side. Let us work this method out more rigorously. Take

We rewrite the equation as follows. Note that is a function of and so is !

We integrate both sides with respect to .

We use the change of variables formula.

And we are done.

#### 1.3.2Implicit solutions

It is clear that we might sometimes get stuck even if we can do the integration. For example, take the separable equation

We separate variables,

We integrate to get

or perhaps the easier looking expression (where )

It is not easy to ﬁnd the solution explicitly as it is hard to solve for . We, therefore, leave the solution in this form and call it an implicit solution. It is still easy to check that an implicit solution satisﬁes the diﬀerential equation. In this case, we diﬀerentiate with respect to to get

It is simple to see that the diﬀerential equation holds. If you want to compute values for , you might have to be tricky. For example, you can graph as a function of , and then ﬂip your paper. Computers are also good at some of these tricks.

We note that the above equation also has the solution . The general solution is together with . These outlying solutions such as are sometimes called singular solutions.

#### 1.3.3Examples

Example 1.3.2: Solve , .

First factor the right hand side to obtain

Separate variables, integrate, and solve for .

Now solve for the initial condition, to get (or , etc…). The solution we are seeking is, therefore,

Example 1.3.3: Bob made a cup of coﬀee, and Bob likes to drink coﬀee only once it will not burn him at 60 degrees. Initially at time minutes, Bob measured the temperature and the coﬀee was 89 degrees Celsius. One minute later, Bob measured the coﬀee again and it had 85 degrees. The temperature of the room (the ambient temperature) is 22 degrees. When should Bob start drinking?

Let be the temperature of the coﬀee, and let be the ambient (room) temperature. Newton’s law of cooling states that the rate at which the temperature of the coﬀee is changing is proportional to the diﬀerence between the ambient temperature and the temperature of the coﬀee. That is,

for some constant . For our setup , , . We separate variables and integrate (let and denote arbitrary constants)

That is, . We plug in the ﬁrst condition: , and hence . So . The second condition says . Solving for we get . Now we solve for the time that gives us a temperature of 60 degrees. That is, we solve to get minutes. So Bob can begin to drink the coﬀee at just over 9 minutes from the time Bob made it. That is probably about the amount of time it took us to calculate how long it would take.

Example 1.3.4: Find the general solution to (including singular solutions).

First note that is a solution (a singular solution). So assume that and write

#### 1.3.4Exercises

Exercise 1.3.1: Solve .

Exercise 1.3.2: Solve .

Exercise 1.3.3: Solve , for .

Exercise 1.3.4: Solve , for .

Exercise 1.3.5: Solve . Hint: Factor the right hand side.

Exercise 1.3.6: Solve , where .

Exercise 1.3.7: Solve , for .

Exercise 1.3.8: Find an implicit solution for , for .

Exercise 1.3.9: Find an explicit solution for , .

Exercise 1.3.10: Find an explicit solution for , for .

Exercise 1.3.11: Find an explicit solution for , . It is alright to leave a deﬁnite integral in your answer.

Exercise 1.3.12: Suppose a cup of coﬀee is at 100 degrees Celsius at time , it is at 70 degrees at minutes, and it is at 50 degrees at minutes. Compute the ambient temperature.

Exercise 1.3.101: Solve .

Exercise 1.3.102: Solve , .

Exercise 1.3.103: Find an implicit solution for , .

Exercise 1.3.104: Find an explicit solution to , .

Exercise 1.3.105: Find an implicit solution to .

Exercise 1.3.106: Take Example 1.3.3 with the same numbers: 89 degrees at , 85 degrees at , and ambient temperature of 22 degrees. Suppose these temperatures were measured with precision of degrees. Given this imprecision, the time it takes the coﬀee to cool to (exactly) 60 degrees is also only known in a certain range. Find this range. Hint: Think about what kind of error makes the cooling time longer and what shorter.

Exercise 1.3.107: A population of rabbits on an island is modeled by , where the independent variable is time in months. At time , there are 40 rabbits on the island. a) Find the solution to the equation with the initial condition. b) How many rabbits are on the island in 1 month, 5 months, 10 months, 15 months (round to the nearest integer).