### Key Concepts

## 5.1 Approximating Areas

- The use of sigma (summation) notation of the form $\sum _{i=1}^{n}{a}_{i}$ is useful for expressing long sums of values in compact form.
- For a continuous function defined over an interval $\left[a,b\right],$ the process of dividing the interval into
*n*equal parts, extending a rectangle to the graph of the function, calculating the areas of the series of rectangles, and then summing the areas yields an approximation of the area of that region. - The width of each rectangle is $\text{\Delta}x=\frac{b-a}{n}.$
- Riemann sums are expressions of the form $\sum _{i=1}^{n}f\left({x}_{i}^{*}\right)\text{\Delta}x},$ and can be used to estimate the area under the curve $y=f\left(x\right).$ Left- and right-endpoint approximations are special kinds of Riemann sums where the values of $\left\{{x}_{i}^{*}\right\}$ are chosen to be the left or right endpoints of the subintervals, respectively.
- Riemann sums allow for much flexibility in choosing the set of points $\left\{{x}_{i}^{*}\right\}$ at which the function is evaluated, often with an eye to obtaining a lower sum or an upper sum.

## 5.2 The Definite Integral

- The definite integral can be used to calculate net signed area, which is the area above the
*x*-axis less the area below the*x*-axis. Net signed area can be positive, negative, or zero. - The component parts of the definite integral are the integrand, the variable of integration, and the limits of integration.
- Continuous functions on a closed interval are integrable. Functions that are not continuous may still be integrable, depending on the nature of the discontinuities.
- The properties of definite integrals can be used to evaluate integrals.
- The area under the curve of many functions can be calculated using geometric formulas.
- The average value of a function can be calculated using definite integrals.

## 5.3 The Fundamental Theorem of Calculus

- The Mean Value Theorem for Integrals states that for a continuous function over a closed interval, there is a value
*c*such that $f\left(c\right)$ equals the average value of the function. See The Mean Value Theorem for Integrals. - The Fundamental Theorem of Calculus, Part 1 shows the relationship between the derivative and the integral. See Fundamental Theorem of Calculus, Part 1.
- The Fundamental Theorem of Calculus, Part 2 is a formula for evaluating a definite integral in terms of an antiderivative of its integrand. The total area under a curve can be found using this formula. See The Fundamental Theorem of Calculus, Part 2.

## 5.4 Integration Formulas and the Net Change Theorem

- The net change theorem states that when a quantity changes, the final value equals the initial value plus the integral of the rate of change. Net change can be a positive number, a negative number, or zero.
- The area under an even function over a symmetric interval can be calculated by doubling the area over the positive
*x*-axis. For an odd function, the integral over a symmetric interval equals zero, because half the area is negative.

## 5.5 Substitution

- Substitution is a technique that simplifies the integration of functions that are the result of a chain-rule derivative. The term ‘substitution’ refers to changing variables or substituting the variable
*u*and*du*for appropriate expressions in the integrand. - When using substitution for a definite integral, we also have to change the limits of integration.

## 5.6 Integrals Involving Exponential and Logarithmic Functions

- Exponential and logarithmic functions arise in many real-world applications, especially those involving growth and decay.
- Substitution is often used to evaluate integrals involving exponential functions or logarithms.

## 5.7 Integrals Resulting in Inverse Trigonometric Functions

- Formulas for derivatives of inverse trigonometric functions developed in Derivatives of Exponential and Logarithmic Functions lead directly to integration formulas involving inverse trigonometric functions.
- Use the formulas listed in the rule on integration formulas resulting in inverse trigonometric functions to match up the correct format and make alterations as necessary to solve the problem.
- Substitution is often required to put the integrand in the correct form.