Integration and Accumulation of Change: AP Calculus AB/BC Study Guide
Introduction
Welcome to Unit 6, where we dive into the magical world of integration and accumulation of change! Yes, we’re talking about finding out how everything adds up over time. Prepare for some serious 'aha' moments, sprinkled with a bit of math magic. Let’s start with an example to get those gears turning.
Change Over Time
Ever wondered how far your favorite race car driver travels during a threehour race at a constant speed of 65 miles per hour? That’s exactly what we’re about to calculate.
To find out how many miles are accumulated over those three hours, we'll use this nifty equation: [ d = v \cdot t ] where (d) stands for distance, (v) is velocity (speed), and (t) is time.
Plugging in our values: [ d = 65 , \text{miles/hour} \cdot 3 , \text{hours} = 195 , \text{miles} ]
Voilà! Our car zoomed 195 miles. This is accumulation of change in a nutshell: adding up how much something changes over time.
Graphing Change Over Time 📈
Let's visualize it! Imagine a graph where the xaxis represents the time the car spent traveling, and the yaxis shows its speed. The total distance traveled is represented by the area under the curve on this graph. For our example, it’s a simple rectangle because the speed is constant.
But wait! Reallife driving isn't like cruising on an endless highway. Cars speed up, slow down, and make stops. So, what happens when speeds vary?
Example: Fun with Variable Speeds
Picture this: a car leaves its garage, accelerates to 50 miles an hour, drives on the highway, slows down to 20 miles per hour, and finally stops. How far did it travel? This journey isn’t as straightforward, but we can figure it out by looking at a graph with variable speeds.
Visualizing this graph: Here, the shaded area under the wavy, complex curve represents the total distance traveled. It’s no longer a simple rectangle but a patchwork of shapes we need to calculate. Let’s describe it using a piecewise function:
[ f(x) = \begin{cases} 5000x^2 & \text{if } 0 \leq x \leq 0.1 \ 50 & \text{if } 0.1 \leq x \leq 2 \ 5x^4 + 40x + 50 & \text{if } 2 \leq x \leq 2.205 \ 20 & \text{if } 2.205 \leq x \leq 2.75 \ 80x + 240 & \text{if } 2.75 \leq x \leq 3 \ \end{cases} ]
Riemann Sums for the Win! 📊
When analyzing variable speeds, we can use Riemann Sums—a fancy way of saying that we can estimate complex areas under a curve by breaking them into easier, smaller shapes like rectangles.
StepbyStep Riemann Sum

Determine the Number of Rectangles: For our example, we'll use six rectangles.

Graph: Overlay six rectangles on our funky journey graph.

Calculate: Sum the areas of each rectangle: [ \text{Height} \times \text{Base Width} ] Using a base width of (0.5) units each: [ 0.5(0 + 50 + 50 + 50 + 50 + 20) = 110 , \text{miles} ]
Voilà, our car traveled approximately 110 miles. This method includes both over and underestimations, but it’s a great start!
Integral Insights 📈
Riemann sums are grand, but integrals are the real deal—they get the exact area under the curve. Integrals can be thought of as the ultimate antiderivatives, undoing all that differentiation jazz. Here's how to write one for our example: [ \int_{0}^{3} f(x) , dx ]
This notation tells us to find the area under ( f(x) ) from ( x = 0 ) to ( x = 3 ). If you evaluated this definite integral, you'd discover the car traveled exactly 117.44 miles.
Practice Makes Perfect! 📝
Let’s warm up with a practice problem:
Example Problem: Consider a graph of ( f(x) ), representing the vehicle's total distance over time. What’s the average speed?
Draw and divide the graph into multiple shapes:
 First Triangle: Base = 1, Height = 10. Area = ( \frac{1 \cdot 10}{2} = 5 )
 Second Triangle: Base = 1.5, Height = 10. Area = ( \frac{1.5 \cdot (10)}{2} = 7.5 )
 Third Triangle: Base = 0.5, Height = 10. Area = ( \frac{0.5 \cdot 10}{2} = 2.5 )
 Rectangle: Base = 3, Height = 10. Area = ( 3 \cdot 10 = 30 )
 Small Triangle: Base = 1, Height = 20. Area = ( \frac{1 \cdot 20}{2} = 10 )
 Last Triangle: Base = 1, Height = 30. Area = ( \frac{1 \cdot 30}{2} = 15 )
Summing all areas: [ 5  7.5 + 2.5 + 30 + 10 + 15 = 55 ]
The car traveled a total of 55 miles.
Practice Problems and Solutions
Problems:
 A tub fills at 10 gal/min. How long to fill 50 gallons?
 Find the exact area under a given graph geometrically.
 Estimate area using a Riemann sum of ( f(x) = x^2 + 10x ) with five rectangles.
Solutions:
 Time ( t = \frac{50 , \text{gallons}}{10 , \text{gal/minute}} = 5 , \text{minutes} ).
 Compute trapezoid areas, sum them.
 Using Riemann sums, estimate areas.
Conclusion
Bravo! You’ve grasped the concept of accumulation of change, graphing it, and approximating areas using Riemann sums. You’re all set to delve deeper into integrals and ace those calculus exams. 🚗💨