Finding General Solutions Using Separation of Variables: AP Calculus AB/BC Study Guide
Introduction
Hey there, future mathematicians and enthusiasts of elegant equations! 🌟 Today, we're diving into the fantastically fun world of differential equations and learning how to find their general solutions using separation of variables. Grab your calculator and your curiosity—let's make calculus as exhilarating as solving a mystery novel! 🔍📐
Solving Differential Equations
A differential equation isn't just math with fancy squiggles; it's the key to predicting how things change, like how fast your savings grow in your piggy bank or the rate at which your coffee cools down (crucial, I know). Essentially, a differential equation relates a function with its derivatives (the rate of change). The goal here is to separate variables and find a continuous function that satisfies the differential equation.
So, how do we go from equation to solution? These steps will take us through the thrilling (yes, thrilling!) journey:
Separable Differential Equations
Separable differential equations are the kind you can split into functions of one variable each. They usually show up as:
[ \frac{dy}{dx} = g(x) \cdot h(y) ]
Where ( g(x) ) is a function of ( x ) and ( h(y) ) is a function of ( y ). Think of it as the algebraic equivalent of cancelling out the background noise to hear your favorite song.
Steps to Solve Separable Differential Equations

Separate Variables: Divide and conquer! Rewrite the differential equation so that every term with ( y ) is on one side and every term with ( x ) is on the other. It’s like organizing a party – put all the snacks in the kitchen and all the people in the living room.
[ \frac{dy}{h(y)} = g(x) \cdot dx ]

Integrate Both Sides: Break out the integration skills! Integrate both sides with respect to their respective variables. Don't forget to add the constant of integration (( C )), which is like the extra flair you add to your math 'party.' 🎉

Solve for ( y ): Solve the resulting equation for ( y ) if you can. It’s your moment to shine – time to find out what ( y ) really looks like.

Incorporate Initial Conditions (if given): If you’re given initial conditions, use them to find the specific value of the constant ( C ). Think of it as adding the finishing touch to your masterpiece.
Practice Problems (AKA Your Calculus Workout)
Let’s flex those calculus muscles and dive into some practice problems!
Practice Problem #1
Solve the differential equation:
[ \frac{dy}{dx} = \frac{2x}{y} ]
Time to hit the math gym! First, separate the variables:
[ y , dy = 2x , dx ]
Now, integrate both sides:
[ \int y , dy = \int 2x , dx ]
After integrating, you get:
[ \frac{1}{2} y^2 = x^2 + C ]
To solve for ( y ), simply:
[ y^2 = 2x^2 + 2C ]
[ y = \pm \sqrt{2x^2 + 2C} ]
So, the general solution is:
[ y = \pm \sqrt{2x^2 + 2C} ]
Voilà! 🎇
Practice Problem #2
Solve the differential equation:
[ \frac{dy}{dx} = x^2 y ]
Let's separate variables:
[ \frac{dy}{y} = x^2 , dx ]
Time to integrate both sides:
[ \int \frac{1}{y} , dy = \int x^2 , dx ]
After integrating, we have:
[ \ln y = \frac{1}{3} x^3 + C ]
To solve for ( y ):
[ y = e^{\frac{1}{3} x^3 + C} = e^C \cdot e^{\frac{1}{3} x^3} ]
Expressing the constant ( e^C ) as ( C ), we get:
[ y = C e^{\frac{1}{3} x^3} ]
Hence, the general solution is:
[ y = \pm C e^{\frac{1}{3} x^3} ]
Well done! 👏
Key Terms to Review
 Antidifferentiation: Finding the original function when given its derivative. Reverseengineering at its finest!
 Constant of Integration: The arbitrary constant added when finding indefinite integrals.
 Differential Equation: An equation that relates derivatives of a function to the function itself.
 Explicit Solution: Directly gives the value of a variable (as opposed to playing hard to get).
 Implicit Solution: Satisfies the equation without explicitly solving for the variable.
 Initial Conditions: The starting details you need to nail down the specific solution.
 Separable Differential Equation: Can be split so each variable is on one side, solving the equation like a pro.
Closing Time!
Congratulations, math champion! 🎉 You’ve navigated through the exciting world of separable differential equations. By mastering these, you're unlocking the power to model and solve realworld problems with style and precision. Keep practicing those problems to solidify your understanding, and you'll be ready to tackle the AP Calculus exam with the confidence of a mathematician extraordinaire!
Now, get out there and show those differential equations who’s boss. Happy studying! 🍀📚