The Definite Integral: Learn It 3

Properties of the Definite Integral

The properties of indefinite integrals apply to definite integrals as well. Definite integrals also have properties related to the limits of integration. These properties, along with the rules of integration that we examine later, help us manipulate expressions to evaluate definite integrals.

Properties of the Definite Integral

  1. [latex]\displaystyle\int_a^a f(x) dx = 0[/latex]

    If the limits of integration are the same, the integral is just a line and contains no area.

  2. [latex]\displaystyle\int_b^a f(x) dx = −\displaystyle\int_a^b f(x) dx[/latex]

    If the limits are reversed, then place a negative sign in front of the integral.

  3. [latex]\displaystyle\int_a^b [f(x)+g(x)] dx = \displaystyle\int_a^b f(x) dx + \displaystyle\int_a^b g(x) dx[/latex]

    The integral of a sum is the sum of the integrals.

  4. [latex]\displaystyle\int_a^b [f(x)-g(x)] dx = \displaystyle\int_a^b f(x) dx - \displaystyle\int_a^b g(x) dx[/latex]

    The integral of a difference is the difference of the integrals.

  5. [latex]\displaystyle\int_a^b cf(x) dx= c \displaystyle\int_a^b f(x) dx[/latex]

    for constant [latex]c[/latex]. The integral of the product of a constant and a function is equal to the constant multiplied by the integral of the function.

  6. [latex]\displaystyle\int_a^b f(x) dx = \displaystyle\int_a^c f(x) dx + \displaystyle\int_c^b f(x) dx[/latex]

    Although this formula normally applies when [latex]c[/latex] is between [latex]a[/latex] and [latex]b[/latex], the formula holds for all values of [latex]a[/latex], [latex]b[/latex], and [latex]c[/latex], provided [latex]f(x)[/latex] is integrable on the largest interval.

Use the properties of the definite integral to express the definite integral of [latex]f(x)=-3x^3+2x+2[/latex] over the interval [latex][-2,1][/latex] as the sum of three definite integrals.

If it is known that [latex]\displaystyle\int_0^8 f(x) dx = 10[/latex] and [latex]\displaystyle\int_0^5 f(x) dx = 5[/latex], find the value of [latex]\displaystyle\int_5^8 f(x) dx[/latex].

Comparison Properties of Integrals

A picture can sometimes tell us more about a function than the results of computations. Comparing functions by their graphs as well as by their algebraic expressions can often give new insight into the process of integration.

Intuitively, we might say that if a function [latex]f(x)[/latex] is above another function [latex]g(x)[/latex], then the area between [latex]f(x)[/latex] and the [latex]x[/latex]-axis is greater than the area between [latex]g(x)[/latex] and the [latex]x[/latex]-axis. This is true depending on the interval over which the comparison is made. The properties of definite integrals are valid whether [latex]ab[/latex].

The following properties, however, concern only the case [latex]a \le b[/latex], and are used when we want to compare the sizes of integrals.

Comparison Theorem

  1. If [latex]f(x) \ge 0[/latex] for [latex]a \le x \le b[/latex], then
    [latex]\displaystyle\int_a^b f(x) dx \ge 0[/latex].
  2. If [latex]f(x) \ge g(x)[/latex] for [latex]a \le x \le b[/latex], then
    [latex]\displaystyle\int_a^b f(x) dx \ge \displaystyle\int_a^b g(x) dx[/latex].
  3. If [latex]m[/latex] and [latex]M[/latex] are constants such that [latex]m \le f(x) \le M[/latex] for [latex]a \le x \le b[/latex], then
    [latex]m(b-a) \le \displaystyle\int_a^b f(x) dx \le M(b-a)[/latex].

Compare [latex]f(x)=\sqrt{1+x^2}[/latex] and [latex]g(x)=\sqrt{1+x}[/latex] over the interval [latex][0,1][/latex].