A Differential Equation is an equation v a duty and one or more of its derivatives:

Example: one equation through the function y and its derivative dy dx


Separation the Variables have the right to be used when:

All the y terms (including dy) have the right to be relocated to one side of the equation, and

All the x state (including dx) to the various other side.

You are watching: Solve the given differential equation by separation of variables.


Three Steps:

Step 1 relocate all the y state (including dy) to one side of the equation and all the x terms (including dx) to the various other side.Step 2 Integrate one side with respect come y and also the various other side v respect to x. Don"t forget "+ C" (the constant of integration).Step 3 Simplify

Example: resolve this (k is a constant):

dy dx = ky

Step 1 different the variables by relocating all the y terms to one side of the equation and also all the x state to the various other side:

C is the constant of integration. And also we use D for the other, as it is a different constant.

Step 3 Simplify:

We have actually solved it:

y = cekx

This is a general kind of first order differential equation which turns up in all sorts of unexpected places in real civilization examples.

Example: Rabbits!

The much more rabbits you have the an ext baby hare you will get. Then those rabbits prosper up and have babies too! The population will prosper faster and faster.

The necessary parts that this are:

the populace N at any type of time tthe expansion rate rthe population"s rate of adjust dN dt

The price of readjust at any kind of time equals the expansion rate time the population:

dN dt = rN

But hey! This is the very same as the equation we just solved! It just has different letters:

N instead of yt rather of xr rather of k

So we can jump to a solution:

N = cert

And here is an example, the graph of N = 0.3e2t:

Exponential Growth

There are various other equations that follow this pattern together as consistent compound interest.

More Examples

OK, on come some various examples the separating the variables:

Example: fix this:

dydx = 1y

Step 1 separate the variables by moving all the y terms to one side of the equation and all the x state to the various other side:

Multiply both political parties by dx:dy = (1/y) dx
Multiply both sides by y: y dy = dx

Step 2 Integrate both political parties of the equation separately:

Put the integral authorize in front:∫ y dy = ∫ dx
Integrate every side:(y2)/2 = x + C

We integrated both political parties in the one line.

We additionally used a shortcut of just one consistent of integration C. This is perfect OK together we could have +D ~ above one, +E ~ above the other and just say the C = E−D.

Step 3 Simplify:

Multiply both sides by 2: y2 = 2(x + C)
Square source of both sides:y = ±√(2(x + C))

Note: This is no the exact same as y = √(2x) + C, since the C was added before we took the square root. This wake up a lot with differential equations. Us cannot just include the C at the finish of the process. The is added when law the integration.

We have solved it:

y = ±√(2(x + C))

A more difficult example:

Example: fix this:

dydx = 2xy1+x2

Step 1 separate the variables:

Multiply both political parties by dx, division both political parties by y:

1y dy = 2x1+x2dx

Step 2 Integrate both political parties of the equation separately:

1y dy = ∫2x1+x2dx

The left side is a an easy logarithm, the best side can be integrated using substitution:

Let u = 1 + x2, therefore du = 2x dx:∫1y dy = ∫1udu
Integrate:ln(y) = ln(u) + C
Then us make C = ln(k):ln(y) = ln(u) + ln(k)
So us can acquire this:y = uk
Now placed u = 1 + x2 earlier again:y = k(1 + x2)

Step 3 Simplify:

It is currently as straightforward as can be. We have solved it:

y = k(1 + x2)

An also harder example: the renowned Verhulst Equation

Example: rabbit Again!

Remember our development Differential Equation:

dNdt = rN

Well, that expansion can"t go on forever as they will shortly run the end of accessible food.

See more: Joseph Beuys And How To Explain Pictures To A Dead Hare : Analysis

A guy dubbed Verhulst included k (the maximum populace the food can support) come get:

dNdt = rN(1−N/k)

The Verhulst Equation

Can this be solved?

Yes, with the assist of one cheat ...

Step 1 different the variables:

Multiply both political parties by dt:dN = rN(1−N/k) dt
Divide both sides by N(1-N/k):1N(1−N/k)dN = r dt

Step 2 Integrate:

1N(1−N/k)dN = ∫ r dt

Hmmm... The left next looks tough to integrate. In fact it have the right to be done with a small trick from Partial fractions ... We rearrange it choose this:

We begin with this:1N(1−N/k)
Multiply top and bottom by k:kN(k−N)
Now here is the trick, add N and also −N come the top:N+k−NN(k−N)
and separation it into two fractions:NN(k−N) + k−NN(k−N)
Simplify every fraction:1k−N + 1N

Now that is a lot much easier to solve. We can incorporate each ax separately, favor this:

Our full equation is now:∫1k−NdN + ∫1NdN = ∫ r dt
Integrate:−ln(k−N) + ln(N) = rt + C

(Why go that come to be minus ln(k−N)? because we room integrating with respect come N.)

Step 3 Simplify:

Negative of all terms:ln(k−N) − ln(N) = −rt − C
Combine ln():ln((k−N)/N) = −rt − C
Now take it exponents ~ above both sides:(k−N)/N = e−rt−C
different the powers of e:(k−N)/N = e−rt e−C
e−C is a constant, we deserve to replace it v A:(k−N)/N = Ae−rt

We are gaining close! just a little more algebra to get N ~ above its own:

Separate the portion terms:(k/N)−1 = Ae−rt
Add 1 to both sides:k/N = 1 + Ae−rt
Divide both by k:1/N = (1 + Ae−rt)/k
Reciprocal the both sides:N = k/(1 + Ae−rt)

And we have actually our solution: