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## AP®︎/College Calculus AB

### Unit 7: Lesson 5

Finding general solutions using separation of variables- Separable equations introduction
- Addressing treating differentials algebraically
- Separable differential equations
- Separable differential equations: find the error
- Worked example: separable differential equations
- Separable differential equations
- Worked example: identifying separable equations
- Identifying separable equations
- Identify separable equations

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# Identifying separable equations

AP.CALC:

FUN‑7 (EU)

, FUN‑7.D (LO)

, FUN‑7.D.1 (EK)

, FUN‑7.D.2 (EK)

To solve a differential equation using separation of variables, we must be able to bring it to the form f, left parenthesis, y, right parenthesis, d, y, equals, g, left parenthesis, x, right parenthesis, d, x where f, left parenthesis, y, right parenthesis is an expression that doesn't contain x and g, left parenthesis, x, right parenthesis is an expression that doesn't contain y.

Not all differential equations are like that. For example, start fraction, d, y, divided by, d, x, end fraction, equals, x, plus, y cannot be brought to the form f, left parenthesis, y, right parenthesis, d, y, equals, g, left parenthesis, x, right parenthesis, d, x no matter how much we try.

In fact, a major challenge with using separation of variables is to identify where this method is applicable. Differential equations that can be solved using separation of variables are called

**separable equations**.So how can we tell whether an equation is separable? The most common type are equations where start fraction, d, y, divided by, d, x, end fraction is equal to a product or a quotient of f, left parenthesis, y, right parenthesis and g, left parenthesis, x, right parenthesis.

For example, start fraction, d, y, divided by, d, x, end fraction, equals, start fraction, start color #11accd, g, left parenthesis, x, right parenthesis, end color #11accd, divided by, start color #ca337c, f, left parenthesis, y, right parenthesis, end color #ca337c, end fraction can turn into start color #ca337c, f, left parenthesis, y, right parenthesis, end color #ca337c, d, y, equals, start color #11accd, g, left parenthesis, x, right parenthesis, end color #11accd, d, x when multiplied by start color #ca337c, f, left parenthesis, y, right parenthesis, end color #ca337c and d, x.

Also, start fraction, d, y, divided by, d, x, end fraction, equals, start color #ca337c, f, left parenthesis, y, right parenthesis, end color #ca337c, start color #11accd, g, left parenthesis, x, right parenthesis, end color #11accd can turn into start fraction, 1, divided by, start color #ca337c, f, left parenthesis, y, right parenthesis, end color #ca337c, end fraction, d, y, equals, start color #11accd, g, left parenthesis, x, right parenthesis, end color #11accd, d, x when divided by start color #ca337c, f, left parenthesis, y, right parenthesis, end color #ca337c and multiplied by d, x.

Here are a few concrete examples:

$\begin{aligned}\dfrac{dy}{dx}&=\overbrace{\maroonD{\sin(y)}}^{\maroonD{f(y)}}\overbrace{\blueD{\ln(x)}}^{\blueD{g(x)}}\\\\\dfrac{1}{\sin(y)}dy&=\ln(x)\,dx\\\\\end{aligned}$ | ||

$\begin{aligned}\dfrac{dy}{dx}&=\dfrac{\overbrace{\blueD{x^3-5x}}^{\blueD{g(x)}}}{\underbrace{\maroonD{e^y}}_{\maroonD{f(y)}}}\\\\e^y\,dy&=(x^3-5x)\,dx\\\\\end{aligned}$ | ||

$\begin{aligned}\dfrac{dy}{dx}&=\dfrac{\overbrace{\maroonD{\sqrt y}}^{\maroonD{f(y)}}}{\underbrace{\blueD{\cos(x)}}_{\blueD{g(x)}}}\\\\\dfrac{1}{\sqrt y}dy&=\dfrac{1}{\cos(x)}dx\end{aligned}$ |

Other equations must be slightly manipulated before they are in the form start fraction, d, y, divided by, d, x, end fraction, equals, f, left parenthesis, y, right parenthesis, g, left parenthesis, x, right parenthesis. For example, we need to factor the right-hand side of start fraction, d, y, divided by, d, x, end fraction, equals, x, y, minus, 7, x to bring it to the desired form:

*Want more practice? Try this exercise.*

## Want to join the conversation?

- In the last exercise, It was not immediately clear to me wether the expression on the right side of the equality was either "2^(y
*minus*x)" or "2^(y*times*-x)". How can you tell them apart? Are both of them separable?(1 vote)- I believe it is a convention that a negative sign before a number not enclosed by anything such as brackets, are meant to be interpreted as a minus sign. If they had wanted you to think that the -x was multiplied, then they would explicitly have written it in the form (y · -x) or (y * (-x)).

TLDR; because there is no multiplication sign present, we may confidently assume that the (y-x) is a subtraction and not multiplication.(7 votes)

- what about a partial differential equation where can I find it?(1 vote)