Definite Integrals - Complete Interactive Lesson
Part 1: Riemann Sums
∫ Definite Integrals
Part 1 of 7 — Riemann Sums
| Part | Topic |
|---|---|
| 1 | Riemann Sums |
| 2 | Definite Integral Definition |
| 3 | Properties of Integrals |
| 4 | FTC Part 1 |
| 5 | FTC Part 2 & Net Change |
| 6 | Problem-Solving Workshop |
| 7 | Comprehensive Review |
The Area Problem
How do we find the exact area under a curve? Approximate with rectangles, then take .
Left, Right, and Midpoint Sums
| Type | Sample Point | Formula |
|---|---|---|
| Left () | Left endpoint | |
| Right () | Right endpoint | |
| Midpoint () | Midpoint |
Worked Example
Approximate with using Left Riemann Sum.
. Left endpoints: .
The exact answer is , so is an underestimate.
Over- and Underestimates — Essential AP Skill
| Function Behavior | Left Sum | Right Sum | Midpoint | Trapezoidal |
|---|---|---|---|---|
| Increasing | Under | Over | — | Over |
| Decreasing | Over | Under | — | Under |
| Concave Up | — | — | Under | Over |
| Concave Down | — | — | Over | Under |
Key Concept: For increasing functions, left rectangles miss the top-right corner (under), while right rectangles include extra area (over). The reverse for decreasing.
AP Tip: The AP Exam loves asking "Is this an over- or underestimate?" You MUST justify by citing whether is increasing/decreasing (for L/R) or concave up/down (for M/T).
Riemann Sum Computations 🎯
Trapezoidal Rule
The Trapezoidal Rule averages the Left and Right sums:
Pattern: First and last values appear ONCE; all middle values are DOUBLED.
Equal vs. Unequal Subintervals
With unequal subintervals (common on AP tables), apply the trapezoid formula to each pair:
Worked Example
Trapezoidal approximation of with :
Exact: . Since is concave up, the Trapezoidal sum overestimates. ✓
AP Tip: The trapezoidal rule with table data is one of the most common AP FRQ questions. Practice with unequal subintervals!
Trapezoidal Rule from a Table 🎯
Given the table:
| 0 | 2 | 5 | 8 | 10 | |
|---|---|---|---|---|---|
| 3 | 7 | 11 | 6 | 4 |
Classify each approximation. 🔍
Let be a positive, increasing, concave-up function on .
Compute a Riemann Sum. ✍️
Key Takeaways — Part 1
| Concept | Key Formula |
|---|---|
| Subinterval width | |
| Left Sum | Use left endpoints |
| Right Sum | Use right endpoints |
| Midpoint Sum | Use midpoint of each subinterval |
| Trapezoidal | |
| Over/Under (L/R) | Depends on increasing/decreasing |
| Over/Under (M/T) | Depends on concavity |
Up Next: Part 2 — The Definite Integral.
Part 2: Definite Integral Definition
∫ Definite Integrals
Part 2 of 7 — The Definite Integral
From Riemann Sums to Exact Area
The definite integral is the limit of a Riemann sum as :
Signed Area
| Region | Sign |
|---|---|
| Between curve and -axis, curve ABOVE axis | Positive |
| Between curve and -axis, curve BELOW axis | Negative |
Key Concept: The definite integral gives signed area, not total area. The AP Exam tests this distinction frequently!
Geometric Evaluation
Some integrals can be evaluated using geometry instead of antiderivatives:
| Shape | Integral | Value |
|---|---|---|
| Rectangle | ||
| Triangle | ||
| Trapezoid | ||
| Semicircle |
Verify: ✓
Worked Example — Semicircle
= area of semicircle with =
AP Tip: If you see , think semicircle! No antiderivative needed.
Definite Integral Concepts 🎯
Even and Odd Function Shortcuts
| Type | Definition | Examples | Integral on |
|---|---|---|---|
| Odd | |||
| Even | $x^2, x^4, \cos x, | x |
Worked Example
Split:
AP Tip: Check for symmetry BEFORE computing! It can save significant time on the exam.
Evaluate Definite Integrals 🎯
Interpret each integral. 🔍
Geometric Evaluation ✍️
Key Takeaways — Part 2
| Concept | Key Point |
|---|---|
| Definition | |
| Signed area | Above axis = +, below = − |
| Total area | $\int |
| Odd functions | Integral = 0 on symmetric intervals |
| Even functions | integral from 0 to |
| Geometry | Use triangles, trapezoids, semicircles |
Up Next: Part 3 — Properties of Integrals.
Part 3: Properties of Integrals
∫ Definite Integrals
Part 3 of 7 — Properties of Integrals
Essential Properties
| Property | Formula | Interpretation |
|---|---|---|
| Constant Multiple | Factor constants out | |
| Sum/Difference | Split into separate integrals | |
| Additivity | Combine adjacent intervals | |
| Reversal | Swap limits = flip sign | |
| Zero Width | No interval = no area | |
| Comparison | Bigger function = bigger integral |
Key Concept: Integrals are linear — they respect addition and scalar multiplication. This is used constantly on the AP Exam!
Worked Examples — Given-Value Problems
A common AP question gives you known integral values and asks you to find others.
Given: , , .
| Find | Work | Answer |
|---|---|---|
| (additivity) | ||
| (reversal) | ||
AP Tip: Don't forget that for a constant . Many students miss the constant term!
Apply Integral Properties 🎯
Given: and .
Average Value of a Function
Interpretation: The average height of the function over .
Geometric meaning: . The rectangle with height has the SAME area as the region under the curve.
Worked Example
Find the average value of on .
AP Tip: The Mean Value Theorem for Integrals guarantees there exists a where (if is continuous). They may ask you to find this .
Average Value 🎯
Apply Properties 🔍
Given: , , .
Compute with Properties ✍️
Key Takeaways — Part 3
| Property | Formula |
|---|---|
| Linearity | Constants factor out, sums split |
| Additivity | |
| Reversal | Swap limits → flip sign |
| Average value | |
| Constant integral |
Up Next: Part 4 — FTC Part 1.
Part 4: Fundamental Theorem of Calculus — Part 1
∫ Definite Integrals
Part 4 of 7 — FTC Part 1
The Fundamental Theorem — Part 1
In words: Differentiation undoes integration. If you integrate and then differentiate, you get back.
With the Chain Rule
If the upper limit is a function :
All Variations at a Glance
| Situation | Formula | Key Step |
|---|---|---|
| Upper limit = | Direct application | |
| Upper limit = | Chain Rule | |
| Lower limit = | Reverse limits first | |
| Both limits are functions | Split into two |
AP Tip: FTC Part 1 with the Chain Rule is tested almost every year on the AP Exam. Master this!
Worked Examples
Example 1: ✓ (direct)
Example 2:
, :
Example 3:
Reverse:
Example 4 (Both limits):
Split:
FTC Part 1 🎯
Accumulation Functions
is an accumulation function: it measures how much has "accumulated" from to .
Connecting and
| About | About |
|---|---|
| is increasing | |
| is decreasing | |
| changes sign to | has a local maximum |
| changes sign to | has a local minimum |
| is increasing | is concave up () |
| is decreasing | is concave down () |
| has a local max/min | has an inflection point |
Key Concept: If they give you the graph of , you can determine the behavior of using this same table (since ). This is one of the most common AP graph-analysis questions.
Accumulation Functions 🎯
Let where is continuous.
FTC Part 1 — Match the derivative. 🔍
Compute a specific value. ✍️
Key Takeaways — Part 4
| Concept | Formula |
|---|---|
| FTC Part 1 (basic) | |
| FTC Part 1 (chain) | |
| Variable in lower limit | Reverse limits → negative sign |
| Accumulation starts at 0 | |
| increasing | decreasing |
Up Next: Part 5 — FTC Part 2 & Net Change.
Part 5: Fundamental Theorem of Calculus — Part 2
∫ Definite Integrals
Part 5 of 7 — FTC Part 2 & Net Change
The Evaluation Theorem (FTC Part 2)
Notation: or
Quick Evaluation Examples
| Integral | Antiderivative | Evaluation |
|---|---|---|
Key Fact: You can use ANY antiderivative — so always choose for simplicity.
Evaluate Using FTC Part 2 🎯
Net Change Theorem
The integral of a rate of change gives the NET CHANGE in the original quantity.
Applications Table
| Quantity | Rate | gives... |
|---|---|---|
| Position | Velocity | Displacement |
| Population | Growth rate | Net population change |
| Water in tank | Flow rate | Net change in volume |
| Revenue | Marginal revenue | Net change in revenue |
| Temperature | Rate of change | Net temperature change |
Displacement vs Total Distance
| Concept | Formula | Includes direction? |
|---|---|---|
| Displacement | Yes (can be negative) | |
| Total Distance | $\int | v |
AP Tip: "How far" = total distance (). "What is the displacement" or "change in position" = . Read the question carefully!
Worked Example — Displacement vs Distance
A particle has on .
Displacement:
The particle is 3 units to the LEFT of where it started.
Total Distance: at . Split at the zero:
Key Concept: To compute , find where , split the integral, and negate on intervals where .
Net Change Theorem 🎯
Interpret each integral. 🔍
Net Change Problem ✍️
Key Takeaways — Part 5
| Concept | Formula |
|---|---|
| FTC Part 2 | |
| Net Change | |
| Displacement | (signed) |
| Total Distance | $\int_a^b |
| Position update |
Up Next: Part 6 — Problem-Solving Workshop.
Part 6: Mixed Integration Problems
∫ Definite Integrals
Part 6 of 7 — Problem-Solving Workshop
Combining All Tools
This part brings together everything: Riemann sums, FTC, properties, and applications.
Strategy Guide
| Problem Type | Key Approach |
|---|---|
| Evaluate | Find antiderivative, apply FTC Part 2 |
| FTC Part 1 (+ Chain Rule if needed) | |
| Given integral values | Use linearity and additivity properties |
| Table data | Trapezoidal rule (unequal subintervals) |
| Rate → total change | Net Change Theorem: |
| Even/odd symmetry | Simplify before computing |
| Absolute value | Split at zeros, negate on negative intervals |
AP Tip: On FRQs, always show your setup (the integral expression) before evaluating. Setup points are awarded separately from answer points.
AP-Style Mixed Problems — Set 1 🎯
Absolute Value Integrals — Step by Step
To evaluate :
- Find where (the zeros)
- Determine sign of on each subinterval
- Split the integral at each zero
- Negate on intervals where
Worked Example
at .
- On : , so
- On : , so
Geometric shortcut: forms a V-shape — two right triangles each with base 2 and height 2. Area = . ✓
Mixed Problems — Set 2 🎯
Classify each problem type. 🔍
Trapezoidal Rule from a Table ✍️
| (min) | 0 | 3 | 7 | 10 |
|---|---|---|---|---|
| (gal/min) | 4 | 6 | 10 | 8 |
Key Takeaways — Part 6
| Problem Type | Go-To Tool |
|---|---|
| Evaluate definite integral | FTC Part 2 |
| Differentiate an integral | FTC Part 1 |
| Given values problems | Properties (linearity, additivity) |
| Rate → amount | Net Change Theorem |
| Table data | Trapezoidal Rule |
| Absolute value | Split at zeros |
Up Next: Part 7 — Comprehensive Review.
Part 7: Comprehensive Review
∫ Definite Integrals — Comprehensive Review
Part 7 of 7 — Final Assessment
Complete Summary
| Concept | Key Formula |
|---|---|
| Riemann Sum | |
| Trapezoidal Rule | |
| Definite Integral | |
| FTC Part 1 | |
| FTC Part 1 (chain) | |
| FTC Part 2 | |
| Net Change | |
| Average Value |
Common AP Exam Mistakes
| Mistake | Consequence |
|---|---|
| Forgetting Chain Rule on FTC Part 1 | Missing the factor |
| Confusing displacement with distance | Using when asked for $\int |
| Not splitting at zeros for $\int | f |
| Forgetting for constants | Missing the constant term |
| Wrong Trapezoidal with unequal widths | Using equal when widths vary |
| Not reversing limits when is in lower bound | Sign error |
Final Assessment — Set 1 🎯
Final Assessment — Set 2 🎯
AP-Style Comprehensive 🔍
Final Challenge ✍️
Definite Integrals — Complete! ✅
You have mastered:
| Skill | Parts |
|---|---|
| Riemann Sums (L, R, M, T) | Part 1 |
| Over/Underestimate analysis | Part 1 |
| Signed area & geometry | Part 2 |
| Even/odd symmetry | Part 2 |
| Properties & average value | Part 3 |
| FTC Part 1 (+ Chain Rule) | Part 4 |
| Accumulation functions | Part 4 |
| FTC Part 2 & Net Change | Part 5 |
| Displacement vs distance | Part 5 |
| Mixed problem solving | Parts 6-7 |
AP Exam Checklist
- ✅ Can evaluate definite integrals with FTC Part 2
- ✅ Can differentiate integrals with FTC Part 1 (+ Chain Rule)
- ✅ Can use properties to compute from given values
- ✅ Can apply Trapezoidal Rule to table data
- ✅ Can distinguish displacement from total distance
- ✅ Can analyze accumulation functions from graphs of