# Lec 19 | MIT 18.01 Single Variable Calculus, Fall 2007

TLDRThis MIT OpenCourseWare lecture introduces the Fundamental Theorem of Calculus (FTC), a pivotal concept in calculus, which connects differentiation and integration. The professor explains two versions of FTC, with the primary one stating that the integral of a function from 'a' to 'b' is equal to the difference of its antiderivative evaluated at 'b' and 'a'. The lecture provides intuitive interpretations, such as relating integration to distance traveled, and discusses properties of integrals, including sum rules, constant factoring, and the estimation property. It also covers the change of variables technique in integration, emphasizing the importance of maintaining the correct limits and avoiding sign changes to prevent confusion.

###### Takeaways

- π The Fundamental Theorem of Calculus (FTC) is introduced as a central concept of the course, with two versions, FTC1 and FTC2.
- π FTC1 states that if F'(x) = f(x), then the definite integral from a to b of f(x) dx equals F(b) - F(a), connecting antiderivatives to definite integrals.
- π The new notation F(x) with a subscript a and superscript b is introduced to represent the definite integral from a to b without explicitly writing the function f(x).
- π Examples are provided to illustrate the use of FTC, including calculating areas under curves and volumes, making complex integrations simpler.
- π An intuitive interpretation of FTC is given using the analogy of distance traveled, where the integral of velocity over time equals the net distance.
- π The definite integral is clarified to represent the net area under the curve, considering both above and below the x-axis, not just the area.
- π’ Several properties of integrals are discussed, such as the sum rule, constant factor rule, and the rule for integrals over the same interval.
- β‘οΈ The integral from a to b of f(x) dx is defined to be equal to minus the integral from b to a of f(x) dx, establishing a convention for reverse order.
- π The estimation property is introduced, allowing for inequalities to be integrated, providing bounds on the integral based on the behavior of the function.
- π Change of variables in integration, also known as substitution, is explained with a focus on correctly adjusting the limits of integration based on the substitution.
- β οΈ A warning is given about the pitfalls of substitution when the derivative of the substitution function changes sign, emphasizing the importance of breaking the integral into pieces where the sign is consistent.

###### Q & A

### What is the fundamental theorem of calculus (FTC)?

-The fundamental theorem of calculus states that if F'(x) = f(x), then the integral from a to b of f(x) is equal to F(b) - F(a). It connects differentiation and integration, allowing us to evaluate definite integrals by finding antiderivatives.

### What is the significance of the fundamental theorem of calculus in this course?

-The fundamental theorem of calculus is considered the most important concept in the course because it provides a method to evaluate definite integrals quickly and efficiently using antiderivatives, rather than through more complex techniques like Riemann sums.

### What is the alternative notation for the definite integral introduced in the script?

-The alternative notation for the definite integral is β«[a, b] f(x) dx, which represents the integral of the function f(x) from a to b. This notation is convenient as it does not always require a specific letter to denote the function.

### Can you provide an example of how the fundamental theorem of calculus simplifies the calculation of a definite integral?

-Sure. If we take the function F(x) = x^3/3, its derivative is F'(x) = x^2. By the fundamental theorem of calculus, the integral from 0 to b of x^2 dx is equal to F(b) - F(0), which simplifies to b^3/3 - 0, or just b^3/3.

### What is the geometric interpretation of the definite integral discussed in the script?

-The geometric interpretation of the definite integral is that it represents the signed area under the curve of the function. The area above the x-axis is counted positively, while the area below the x-axis is counted negatively.

### How does the fundamental theorem of calculus relate to the concept of velocity and distance traveled?

-The fundamental theorem of calculus can be interpreted in terms of velocity and distance. If x(t) is the position at time t and v(t) = dx/dt is the velocity, then the integral from a to b of v(t) dt equals the distance traveled from x(a) to x(b), which is x(b) - x(a).

### What is a Riemann sum, and how does it relate to the fundamental theorem of calculus?

-A Riemann sum is a method used to approximate the definite integral by summing the areas of rectangles under the curve of the function. The fundamental theorem of calculus provides a way to calculate the exact value of the integral, which is a more accurate representation of the total distance traveled than the approximation given by the Riemann sum.

### What are some properties of integrals mentioned in the script?

-Some properties of integrals mentioned include: the integral of a sum is the sum of the integrals, constants can be factored out, the integral from a to b and b to c equals the integral from a to c, the integral from a to a is zero, and the integral from a to b is equal to minus the integral from b to a.

### Can you explain the estimation property of integrals and provide an example?

-The estimation property states that if f(x) β€ g(x), then the integral from a to b of f(x) dx is less than or equal to the integral from a to b of g(x) dx. An example given in the script is integrating e^x β₯ 1 for x β₯ 0 from 0 to b, which results in e^b - 1 β₯ b, showing that e^b β₯ 1 + b.

### What is the change of variables technique in integration, and how do you handle the limits when applying it?

-The change of variables technique, also known as substitution, allows you to transform an integral by letting u = u(x) and du = u'(x) dx. When applying this, you must also change the limits of integration according to the new variable u, such that u_1 = u(x_1) and u_2 = u(x_2), ensuring that the derivative u'(x) does not change sign.

###### Outlines

##### π Introduction to the Fundamental Theorem of Calculus

The professor begins by introducing the Fundamental Theorem of Calculus (FTC), emphasizing its importance in calculus. The FTC connects differentiation and integration, stating that if a function F is the antiderivative of f(x), then the definite integral from a to b of f(x) is F(b) - F(a). The professor explains two notations for this theorem and provides an example using the function F(x) = x^3/3 to illustrate how the FTC simplifies the calculation of integrals.

##### π Examples and Applications of the Fundamental Theorem

This section continues with practical examples to demonstrate the FTC's utility. The professor calculates the area under a curve for a function like sin(x) from 0 to Ο, highlighting the ease with which the FTC can handle problems that would be complex with Riemann sums. The discussion also includes the integral of x^100 from 0 to 1, showcasing the theorem's versatility. The professor then offers an intuitive interpretation of the FTC relating to distance traveled, using the concepts of position and velocity.

##### π Intuitive Interpretation and Riemann Sums

The professor provides an intuitive interpretation of the FTC using the analogy of distance and speed. By considering x(t) as position and x'(t) as speed, the FTC is shown to equate the integral of velocity over a time interval to the change in position. The connection to Riemann sums is also discussed, with a thought experiment illustrating how checking speed at one-second intervals can approximate the total distance traveled, aligning with the FTC's principles.

##### π Extending Integration to Handle Negative Values

The professor extends the concept of integration to include negative values, emphasizing that the formulas remain the same but the interpretation changes. The definite integral is redefined to account for areas above and below the x-axis, with a clear example using the sine function from 0 to 2Ο. The integral's result is zero, demonstrating how positive and negative areas cancel each other out, reflecting no net change in position.

##### π Properties of Integrals and Their Explanation

The focus shifts to the properties of integrals, starting with the additive property, which allows the integration of a sum of functions to be broken into separate integrals. The professor also discusses the historical reasoning behind the integral symbol's design, relating it to the sum symbol. Constant factors can be pulled out of integrals, and the properties are established to maintain consistency with the FTC.

##### π Understanding Integral Properties and Their Implications

The professor explains additional properties of integrals, such as the ability to integrate over intervals in any order due to a new definition, and the fact that the integral from a to a is zero. The fifth property introduces the concept of integrals in reverse order, defining it as the negative of the integral in the forward direction. These properties are crucial for maintaining consistency within calculus operations.

##### π The Estimation Property and Its Demonstration

The sixth property, estimation, is introduced, which allows for comparing the integrals of two functions where one is less than or equal to the other. The professor illustrates this with an example, integrating the inequality e^x >= 1 from 0 to b, resulting in e^b >= 1 + b. The importance of the direction of integration (a < b) is highlighted, and the potential for error when dealing with negative values is noted.

##### π§© Change of Variables in Integration and Its Subtleties

The final topic is the change of variables in integration, also known as substitution. The professor outlines the process and emphasizes the importance of maintaining the correct limits when substituting variables. A common mistake is pointed out, where ignoring the sign change in the derivative can lead to incorrect results. The professor advises breaking the integral into pieces where the derivative maintains a consistent sign to avoid confusion.

###### Mindmap

###### Keywords

##### π‘Fundamental Theorem of Calculus (FTC)

##### π‘Antiderivative

##### π‘Definite Integral

##### π‘Riemann Sum

##### π‘Integration by Substitution

##### π‘Velocity and Distance

##### π‘Properties of Integrals

##### π‘Estimation

##### π‘Geometric Interpretation

##### π‘Sign Change

###### Highlights

Introduction to the fundamental theorem of calculus (FTC), a key concept in the course.

FTC states the integral of a function from a to b is equal to the difference of its antiderivative evaluated at those points.

Explanation of the antiderivative notation and its connection to definite integrals.

Introduction of a new notation for definite integrals without specifying a function symbol.

Example calculation using FTC to find the integral of x^2 from a to b.

Demonstration of the FTC's utility in quickly solving integrals compared to Riemann sums.

Application of FTC to calculate the area under one hump of the sine curve from 0 to Ο.

Clarification of the geometric interpretation of definite integrals as areas above and below the x-axis.

Discussion on the properties of integrals, including sum and constant factorization.

Explanation of the integral's property regarding the addition of areas from separate intervals.

Rule that the integral from a to a is zero, highlighting the absence of horizontal movement.

Definition of the integral from a to b as the negative of the integral from b to a.

Illustration of the estimation property, showing that if one function is less than another, its integral is also less.

Application of the estimation property to prove inequalities involving exponential functions.

Introduction to the change of variables technique in integration, also known as substitution.

Explanation of how to adjust the limits of integration when changing variables.

Warning against using substitution when the derivative of the substitution function changes sign.

Strategy to handle integrals with sign changes by breaking them into pieces with consistent signs.

Example illustrating the mistake of not properly adjusting limits during substitution and how to correct it.

###### Transcripts

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