Exponential Models with Differential Equations: AP Calculus AB/BC Study Guide
Introduction
Hello future calculus wizards! 🌟 Welcome to the exciting world of exponential models and differential equations, where math meets the real world in surprising ways. Imagine understanding how quickly your latest TikTok video goes viral, or predicting the decay of radioactive materials. Grab your metaphorical lab coats, because we’re diving into the exponential jungle!
What is a Differential Equation? 🤔
At the heart of exponential models is the mystical differential equation. Picture it like a detective story where you know the rate at which something is changing—be it a population or a rumor—and you need to figure out the complete story of its growth or decline.
One iconic equation stands at the center of our exploration: [ \frac{dy}{dt} = ky ]
Don’t worry, it’s not as scary as it looks! Here’s the breakdown:
 ( \frac{dy}{dt} ): This represents the rate of change over time, like how many people are catching on to the latest dancing trend on TikTok every minute.
 ( k ): This is our constant friend that tells us how flirty or how shy the change is. A positive ( k ) means things are speeding up, and a negative ( k ) means things are slowing down. Think of ( k ) as the hype factor! 🎉
Solving the Differential Equation 🔍
To unravel this mystery, let’s follow the trail:

Start with the Differential Equation: We’ll kick things off with ( \frac{dy}{dt} = ky ).

Separate the Variables: Isolate the variables by shuffling ( y ) to one side and ( t ) to the other: [ \frac{1}{y} , dy = k , dt ]

Integrate Both Sides: Integrate these buddies to bring them together: [ \int \frac{1}{y} , dy = \int k , dt ]
This gives us: [ \lny = kt + C ]

Solve for ( y ): To make it sparkle, exponentiate both sides and simplify: [ y = e^{C} e^{kt} ]
Let’s call ( e^C ) our initial value ( y_0 ): [ y = y_0 e^{kt} ]
And voilà! We have our elegant exponential model. This model tells us that ( y ) (the thing we care about, like a population) changes over time based on its initial value ( y_0 ), ( e ) (Euler’s magic number), and the combined effects of the rate ( k ) and time ( t ).
Walking Through an Exponential Model 🎯
Let’s flex your new skills with a practice problem:
Problem: A quaint town starts with a population of 2,000. Due to new job opportunities, the population is increasing exponentially. After 3 years, the population reaches 3,000. What will the population be after 10 years?
StepbyStep Solution:
 Initial Population: ( y_0 = 2,000 )
 Population after 3 Years: ( y = 3,000 ) when ( t = 3 )
Start with our exponential model: [ y = y_0 e^{kt} ]
Plugging in what we know: [ 3,000 = 2,000 \cdot e^{3k} ]
Solve for ( k ): [ \frac{3,000}{2,000} = e^{3k} ] [ 1.5 = e^{3k} ] [ \ln(1.5) = 3k ] [ k = \frac{\ln(1.5)}{3} \approx 0.13516 ]
Now use ( k ) to find the population after 10 years: [ y = 2,000 \cdot e^{0.13516 \cdot 10} ] [ y = 2,000 \cdot e^{1.3516} ] [ y \approx 2,000 \cdot 3.8637 \approx 7,727 ]
The population will be approximately 7,727 after 10 years. 👏
Try It Yourself ✏️
Give these problems a whirl!

Drug Decay: A dose of 200 mg of a drug is administered, leaving 127.3 mg after 3 hours. Write the equation modeling the drug decay.

Bacteria Growth: Bacteria in a culture double every five hours. How long until the number quadruples?
Hint: Even though you don’t have explicit numbers, use ratios to set up and solve the exponential growth equations.
Conclusion 🥳
Exponential models empower us to predict superfast growth or decline in our dynamic world. From viral videos to biological processes, these tools are essential in unlocking the secrets of how things change. Master these concepts, not just to ace your AP Calculus exam, but to become a true mathematician who sees the patterns in the world around you.
Now, go forth and let your exponential growth in knowledge begin! 🚀
Key Terms to Review
 Euler's Number (e): An irrational number approximately equal to 2.71828, crucial in continuous growth/decay.
 Exponential Growth Model: A mathematical representation of a process increasing at an accelerating rate over time, characterized by a constant positive growth factor.
Happy Calculating!