Chapter Study Guide: The Definite Integral — Calculus II: Integral Calculus - Integration, Series, and Parametric Equations Study Notes | BrainyBee

Learning Objectives

1.2.1 State the definition of the definite integral.
1.2.2 Explain the terms integrand, limits of integration, and variable of integration.
1.2.3 Explain when a function is integrable.
1.2.4 Describe the relationship between the definite integral and net area.
1.2.5 Use geometry and the properties of definite integrals to evaluate them.
1.2.6 Calculate the average value of a function.

Key Terms & Glossary

Definite Integral: A formalization of the area under a curve, evaluated between two specific limits, yielding a specific numeric value.
Integrable Function: A function $f(x) for which the limit of its Riemann sum exists over a given interval [a, b]$ .
Integrand: The function $f(x)$ that is being integrated.
Limits of Integration: The boundaries $a$ (lower) and $b$ (upper) over which the integral is evaluated.
Variable of Integration: A dummy variable (e.g., $dx$ ) that specifies with respect to which variable the integration is being performed.

The "Big Idea"

The definite integral generalizes the concept of finding the area under a curve. While Riemann sums initially required functions to be continuous and non-negative, the definite integral lifts these restrictions to model real-world scenarios more robustly. Unlike an indefinite integral (which represents a family of functions), a definite integral results in a specific number representing the net area between the function and the $x$ -axis.

[!NOTE] Notation History: The integration symbol $\int$ is an elongated "S", introduced by Gottfried Wilhelm Leibniz, representing "sigma" or summation.

Formula / Concept Box

Property	Formula	Description
Zero Interval	$\int_a^a f(x) dx = 0$	An integral over a point has no width, hence zero area.
Reversed Limits	$\int_b^a f(x) dx = -\int_a^b f(x) dx$	Reversing the direction of integration changes the sign.
Sum/Difference	$\int_a^b [f(x) \pm g(x)] dx = \int_a^b f(x) dx \pm \int_a^b g(x) dx$	Integrals can be split over addition and subtraction.
Constant Multiple	$\int_a^b c \cdot f(x) dx = c \int_a^b f(x) dx$	Constants can be pulled outside the integral.
Interval Additivity	$\int_a^b f(x) dx + \int_b^c f(x) dx = \int_a^c f(x) dx$	Adjacent areas can be combined.

Hierarchical Outline

1. The Definite Integral
- 1.1. Core Definition
  - Generalization of the Riemann Sum
  - Transitioning from limits of sums ( $\lim_{n \to \infty}$ ) to exact area
- 1.2. Anatomy of the Notation
  - Integral Symbol ( $\int$ )
  - Limits of Integration ( $a$ to $b$ )
  - Integrand ( $f(x)$ )
  - Variable of integration ( $dx$ )
- 1.3. Evaluation Strategies
  - Using Geometric Formulas (circles, triangles, rectangles)
  - Applying Definite Integral Properties

Visual Anchors

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Definition-Example Pairs

Term: Limits of Integration
- Definition: The specific boundary values on the $x$ -axis over which the area is calculated.
- Real-World Example: Calculating the total water flowed into a tank from $t=2$ hours to $t=5 hours. The limits of integration are a=2$ and $b=5$ .
Term: Variable of Integration
- Definition: A dummy variable showing the variable of respect for integration.
- Real-World Example: $\int_0^1 f(t) dt$ and $\int_0^1 f(x) dx$ yield the exact same numerical result. It simply marks the "axis" of calculation.

Worked Examples

Example 1: Using Geometric Formulas to Calculate Definite Integrals

Problem: Use the formula for the area of a circle to evaluate the area represented by the integral of a semicircle function with radius 3, from $x = -3$ to $x = 3$ .

Step-by-Step Breakdown:

Identify the Geometry: The function describes a semicircle centered at the origin with radius $r = 3$ .
Determine the Area Formula: The total area of a full circle is $A = \pi r^2$ .
Adapt for Semicircle: Since the integral bounds cover exactly the upper half of the circle, we need half the area: $A_{semi} = \frac{1}{2} \pi r^2$ .
Substitute and Solve: $\text{Area} = \frac{1}{2} \pi (3)^2 = \frac{9\pi}{2}$
Conclusion: The definite integral evaluates to $\frac{9\pi}{2}$ .

Example 2: Using the Properties of the Definite Integral

Problem: Express the definite integral $\int_a^b [3f(x) + 2g(x)] dx$ as the sum of separate definite integrals.

Step-by-Step Breakdown:

Apply Sum Rule: Split the integral at the addition sign. $\int_a^b 3f(x) dx + \int_a^b 2g(x) dx$
Apply Constant Multiple Rule: Pull the constants (3 and 2) outside their respective integrals. $3\int_a^b f(x) dx + 2\int_a^b g(x) dx$

Comparison Tables

Feature	Definite Integral	Indefinite Integral
Notation	$\int_a^b f(x) dx$	$\int f(x) dx$
Output Type	A specific Number (Value)	A family of Functions
Represents	Net area under a curve	Antiderivatives
Limits of Integration	Required ( $a$ and $b$ )	None

Checkpoint Questions

How does a definite integral fundamentally differ from an indefinite integral in terms of its final output?
If $\int_1^5 f(x) dx = 10$ , what is the value of $\int_5^1 f(x) dx$ and which property defines this?
Why is the variable of integration referred to as a "dummy variable"?
How can you evaluate a definite integral if you do not know the algebraic integration rules yet?