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Applications of Definite Integrals

Last Updated : 29 Jul, 2024
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Definite Integrals are used to find areas of the complex curve, volumes of irregular shapes, and other things. Definite Integrals are defined by, let us take p(x) to be the antiderivative of a continuous function f(x) defined on [a, b] then, the definite integral of f(x) over [a, b] is denoted by\int\limits_{a}^{b}f(x)dx  and is equal to [p(b) - p(a)].

\bold{\int\limits_{a}^{b}f(x)dx}   = p(b) - p(a)

The numbers a and b are called the limits of integration where a is called the lower limit and b is called the upper limit. The interval [a, b] is called the interval of the integration.

Note

  • Constant of Integration is not included in the evaluation of the definite integral.
  • \bold{\int\limits_{a}^{b}f(x)dx}   is read as "integral of f(x) from a to b"

How to Evaluate Definite Integrals?

To find the definite integral of f(x) over interval [a, b] i.e.,\int\limits_{a}^{b}f(x)dx   we have following steps:

Step 1: Find the indefinite integral ∫f(x) dx.

Step 2: Evaluate p(a) and p(b) where, p(x) is the antiderivative of f(x), p(a) is the value of antiderivative at x = a, and p(b) is the value of antiderivative at x = b.

Step 3: Calculate p(b) - p(a).

The value obtained in Step 3 is the desired value of the definite integral.

Properties of Definite Integral

Various Properties of Definite Integral have been discussed below:

Property 1)\bold{\int\limits_{a}^{b}f(x)dx = \int\limits_{a}^{b}f(z)dz}

Proof:

Let p(x) be an antiderivative of f(x). Then,

\frac{d}{dx}   {p(x)} = f(x)

\frac{d}{dz}   {p(z)} = f(z)

\int\limits_{a}^b f(x)dx = \big[p(x)\big]_a^b   = p(b) - p(a) -----------------(i)

\bold{\int\limits_{a}^{b}f(x)dx=\int\limits_{a}^{c}f(x)dx+\int\limits_{c}^{b}f(x)dx}

and\int\limits_{a}^b f(z)dz = \big[p(z)\big]_a^b   = p(b) - p(a) -----------------(ii)

From (i) and (ii)

\bold{\int\limits_{a}^{b}f(x)dx= \int\limits_{a}^{b}f(z)dz}

Property 2)\bold{\int\limits_{a}^{b}f(x)dx=-\int\limits_{b}^{a}f(x)dx}

If the limits of the definite integral are interchanged then, its value changes by a minus sign only.

Proof:

Let p(x) be an antiderivative of f(x). Then,

\int\limits_{a}^b f(x)dx   = p(b) - p(a)

and-\int\limits_{b}^a f(x)dx   = -[p(a) - p(b)] = p(b) - p(a)

\bold{\int\limits_{a}^{b}f(x)dx=-\int\limits_{b}^{a}f(x)dx}

Property 3)\bold{\int\limits_{a}^{b}f(x)dx=\int\limits_{a}^{c}f(x)dx+\int\limits_{c}^{b}f(x)dx}   where, a<c<b

Proof:

Let p(x) be the antiderivative of f(x). Then,

\int\limits_{a}^b f(x)dx   = p(b) - p(a) ---------------(i)

\int\limits_{a}^{c}f(x)dx+\int\limits_{c}^{b}f(x)dx   = [p(c) - p(a)] + [p(b) - p(c)] = p(b) - p(a) ---------------(ii)

From (i) and (ii)

\bold{\int\limits_{a}^{b}f(x)dx=\int\limits_{a}^{c}f(x)dx+\int\limits_{c}^{b}f(x)dx}

Property 4)\bold{\int\limits_{0}^{a}f(x)dx=\int\limits_{0}^{a}f(a-x)dx}

Proof:

Let x = a - t. Then, dx = d(a - t) ⇒ dx = -dt

When x=0 ⇒ t = a and x = a ⇒ t = 0

\int\limits_{0}^{a}f(x)dx=-\int\limits_{a}^{0}f(a - t)dt

\int\limits_{0}^{a}f(x)dx=\int\limits_{0}^{a}f(a - t)dt   [Using second property]

\int\limits_{0}^{a}f(x)dx=\int\limits_{0}^{a}f(a - x)dx   [Using first property]

\bold{\int\limits_{0}^{a}f(x)dx=\int\limits_{0}^{a}f(a-x)dx}

Property 5)\bold{\int\limits_{-a}^{a}f(x)dx= \begin{cases} 2\int\limits_{0}^{a}f(x)dx & \text{, if $n$ is even} \\ 0 & \text{, if $n$ is odd} \end{cases}}

Proof:

Using third property

\int\limits_{-a}^{a}f(x)dx=\int\limits_{-a}^{0}f(x)dx+\int\limits_{0}^{a}f(x)dx   ----------(i)

Let x = -t, dx = -dt

Limits: x = -a ⇒ t = a and x = 0 ⇒ t=0

\int\limits_{-a}^{0}f(x)dx=\int\limits_{a}^{0}f(-t)(-dt) =-\int\limits_{a}^{0}f(-t)dt=\int\limits_{0}^{a}f(-t)dt   [By second property]

\int\limits_{-a}^{0}f(x)dx=\int\limits_{0}^{a}f(-x)dx   [By first property] -------------(ii)

From (i) and (ii)

\int\limits_{-a}^{a}f(x)dx=\int\limits_{0}^{a}f(-x)dx+\int\limits_{0}^{a}f(x)dx

\int\limits_{-a}^{a}f(x)dx=\int\limits_{0}^{a}[f(-x)+f(x)]dx

\int\limits_{-a}^{a}f(x)dx = \begin{cases} 2\int\limits_{0}^{a}f(x)dx & , if f(-x) = f(x) \\ 0 & , if f(-x) = -f(x) \end{cases}

\bold{\int\limits_{-a}^{a}f(x)dx= \begin{cases} 2\int\limits_{0}^{a}f(x)dx & \text{, if $n$ is even} \\ 0 & \text{, if $n$ is odd} \end{cases}}

Property 6) If f(x) is a continuous function defined on [0, 2a]\bold{\int\limits_{0}^{2a}f(x)dx= \begin{cases} 2\int\limits_{0}^{a}f(x)dx & , if f(2a - x) = f(x) \\ 0 & , if f(2a - x) = -f(x)\end{cases}}

Proof:

Using third property

\int\limits_{0}^{2a}f(x)dx=\int\limits_{0}^{a}f(x)dx+\int\limits_{a}^{2a}f(x)dx   -----------(i)

Consider

Let x = 2a - t, dx = -d(2a - t) ⇒ dx = -dt

Limits: x = a ⇒ t = a and x = 2a ⇒ t=0

\int\limits_{a}^{2a}f(x)dx=-\int\limits_{a}^{0}f(2a - t)dt

\int\limits_{a}^{2a}f(x)dx=\int\limits_{0}^{a}f(2a - t)dt   [Using second property]

\int\limits_{a}^{2a}f(x)dx=\int\limits_{0}^{a}f(2a - x)dx   [Using first property]

Substituting\int\limits_{a}^{2a}f(x)dx=\int\limits_{0}^{a}f(2a - x)dx   in (i)

\int\limits_{0}^{2a}f(x)dx=\int\limits_{0}^{a}f(x)dx+\int\limits_{0}^{a}f(2a - x)dx = \int\limits_{0}^{a}[f(x) + f(2a - x)]dx

\bold{\int\limits_{0}^{2a}f(x)dx= \begin{cases} 2\int\limits_{0}^{a}f(x)dx & , if f(2a - x) = f(x) \\ 0 & , if f(2a - x) = -f(x)\end{cases}}

Property 7)\bold{\int\limits_{a}^{b}f(x)dx = \int\limits_{a}^{b}f(a + b - x)dx}

Proof:

Let t = a + b - x ⇒ dt = -dx

Limits: x = a, y=b and x = b, y =a

After putting value and limit of t in\int\limits_{a}^{b}f(a + b - x)dx

\int\limits_{a}^{b}f(a + b - x)dx =-\int\limits_{b}^{a}f(t)dt

\int\limits_{a}^{b}f(a + b - x)dx =\int\limits_{a}^{b}f(t)dt   [Using second property]

\int\limits_{a}^{b}f(a + b - x)dx =\int\limits_{a}^{b}f(x)dx   [Using first property]

\bold{\int\limits_{a}^{b}f(x)dx= \int\limits_{a}^{b}f(a + b - x)dx}

Applications of Definite Integrals :

There are numerous definite integral applications across different fields. Some of them include the following : Area under Curves : Definite integrals allow one to calculate the area between a curve and the axis of x within some specified interval. It is used in geometry and physics in determining the size of a region. Total Distance Covered : Integrating the absolute value of the velocity function over an interval would yield a value of total distance traveled by an object. This is very important in Physics and Engineering in the analysis of motion. Work Done by a Force : In physics, the work done by a variable force is calculated by integration of the function of force along a path. This explains the energy transfer and mechanical systems. Total Revenue and Cost Functions : Definite integrals are very important in economic theory for finding the total revenue and total cost functions. Integration of the revenue and cost function within certain intervals is pretty useful in studying profit margins and business decisions. Modeling of Population Growth and Decay : In problems involving biology and ecology, definite integrals are used to study the growth and decay of a population. If the functions of growth or decay in a population are integrated over time, insights into the dynamics and sustainability of the population can be gained. Evaluating Probabilities using Probability Density Functions : In statistics, definite integrals are used for evaluating probabilities, where a probability density function is given. The integration of such functions along certain intervals gives the probability of events happening within that range.

Definite Integral as Limit of Sum

The definite integral of f(x) over the interval [a, b], denoted by\int\limits_a^bf(x)dx   , is defined as the limit of a sum given by:

\int\limits_a^bf(x)dx = \displaystyle \lim_{n\to\infty}\displaystyle \sum_{r=1}^n hf(a + rh) \\ or \\ \displaystyle\lim_{n\to\infty}\displaystyle \sum_{r=0}^{n-1} hf(a + rh)

where nh = b - a

f(x) is said to be integrable over [a, b] if the above two limits exist and are equal.

Challenging Definite Integral

Problem1: Evaluate the definite integral:\int\limits^2_0 x^2[x]dx

Solution:

The integral contains the greatest integer.

For limit 0 to 1 greatest integer function [x] = 0

For limit 1 to 2 greatest integer function [x] = 1

So, we split the above integral into two parts using the following definite integral property.

\int\limits_a^b f(x)dx = \int\limits^c_af(x)dx + \int\limits^b_cf(x)dx

\int\limits_0^2x^2[x]dx = \int\limits_0^1x^2[x]dx+\int\limits_1^2x^2[x]dx

=\int\limits_0^1x^2.0dx+\int\limits_1^2x^2.1dx

\int\limits_0^2x^2[x]dx = 0+\int\limits_1^2x^2dx

\int\limits_0^2x^2[x]dx = \int\limits_1^2x^2dx

\int\limits_0^2x^2[x]dx = \big[\frac{x^3}{3}\big]_1^2

= [8/3] - [1/3]

\int\limits_0^2x^2[x]dx   = 7/3

Problem 2: Evaluate:\int\limits^3_1|x^2-2x| dx

Solution:

The integral contains a mod function.

For limit 1 to 2 let x=1, the function f(x) = x2 - 2x = 1-2 = -1 is negative.

For limit 2 to 3 let x=3, the function f(x) = x2 - 2x = 9-6 = 3 is positive.

So, we split the above limit of integral into two parts using the following definite integral property.

\int\limits_a^b f(x)dx = \int\limits^c_af(x)dx + \int\limits^b_cf(x)dx

\int\limits^3_1|x^2-2x| dx =\int\limits^2_1|x^2-2x| dx +\int\limits^3_2|x^2-2x| dx

\int\limits^3_1|x^2-2x| dx =\int\limits^2_1-(x^2-2x) dx +\int\limits^3_2(x^2-2x) dx

\int\limits^3_1|x^2-2x| dx =\int\limits^2_1(-x^2+2x) dx +\int\limits^3_2(x^2-2x) dx

\int\limits^3_1|x^2-2x| dx =\big[x^2-\frac{x^3}{3}\big]^2_1 +\big[\frac{x^3}{3}-x^2\big]^3_2

\int\limits^3_1|x^2-2x| dx =\big[4-\frac{8}{3}\big]-\big[1-\frac{1}{3}\big] +\big[9-9\big]-\big[\frac{8}{3}-4\big]

\int\limits^3_1|x^2-2x| dx   = 2/3 + 4/3

\int\limits^3_1|x^2-2x| dx   = 2

Problem 3: Evaluate:\int\limits_0^{\pi^2/4}\frac{ cos \sqrt x}{\sqrt x}dx

Solution:

\int\limits_0^{\pi^2/4}\frac{ cos \sqrt x}{\sqrt x}dx

Let √x = t

1/ (2√x) dx = dt

dx/√x = 2dt

Limits: If x=0, t=0 and x=π2/4, t=π/2

\int\limits_0^{\pi^2/4}\frac{ cos \sqrt x}{\sqrt x}dx = 2\int\limits_0^{\pi/2}costdt

\int\limits_0^{\pi^2/4}\frac{ cos \sqrt x}{\sqrt x}dx = 2\big[sint\big]_0^{\pi/2}

\int\limits_0^{\pi^2/4}\frac{ cos \sqrt x}{\sqrt x}dx   = 2(1 - 0)

\int\limits_0^{\pi^2/4}\frac{ cos \sqrt x}{\sqrt x}dx   = 2

Problem 4: Evaluate:\int\limits^{\pi /3}_{\pi/6}\frac{1}{1+\sqrt {cotx}}dx

Solution:

I =\int\limits^{\pi /3}_{\pi/6}\frac{1}{1+\sqrt {cotx}}dx

I =\int\limits^{\pi /3}_{\pi/6}\frac{1}{1+\frac{\sqrt {sinx}}{\sqrt{cosx}}}dx

I =\int\limits^{\pi /3}_{\pi/6}\frac{\sqrt{sinx}}{\sqrt{cosx}+\sqrt {sinx}}dx   ----------(1)

Using property\int\limits^b_af(x)dx = \int\limits^b_af(a+b-x)dx

I =\int\limits^{\pi /3}_{\pi/6}\frac{\sqrt{sin(\frac{\pi}{2}-x)}}{\sqrt{cos(\frac{\pi}{2}-x)}+\sqrt {sin(\frac{\pi}{2}-x)}}dx

I =\int\limits^{\pi /3}_{\pi/6}\frac{\sqrt{cosx}}{\sqrt{sinx}+\sqrt {cosx}}dx   ----------(2)

Adding (1) and (2)

2I =\int\limits^{\pi /3}_{\pi/6}\frac{\sqrt{sinx}}{\sqrt{cosx}+\sqrt {sinx}}dx+\int\limits^{\pi /3}_{\pi/6}\frac{\sqrt{cosx}}{\sqrt{sinx}+\sqrt {cosx}}dx

2I =\int\limits^{\pi /3}_{\pi/6}\frac{\sqrt{sinx}+\sqrt{cosx}}{\sqrt{sinx}+\sqrt {cosx}}dx

2I =\int\limits^{\pi /3}_{\pi/6}1.dx

2I =\big[x\big]^{\pi/3}_{\pi/6}

2I = (π/3) - (π/6)

2I = π/6

I = π/12

Problem 5: Evaluate:\int\limits_0^{\pi/2} |sinx - cosx|dx

Solution:

The integral contains mod function.

sin x - cos x = 0

sin x = cos x

tan x = 1

x = π/4

Between limit 0 to π/4, |sinx - cosx| is negative and between π/4 to π/2, |sinx - cosx| is positive

So, we divide the above limits of integral using the following formula

\int\limits_a^b f(x)dx = \int\limits^c_af(x)dx + \int\limits^b_cf(x)dx

\int\limits_0^{\pi/2} |sinx - cosx|dx = \int\limits_{0}^{\pi/4} -(sinx - cosx)dx+\int\limits_{\pi/4}^{\pi/2} (sinx - cosx)dx

\int\limits_0^{\pi/2} |sinx - cosx|dx = \int\limits_{0}^{\pi/4} (cosx - sinx)dx+\int\limits_{\pi/4}^{\pi/2} (sinx - cosx)dx

\int\limits_0^{\pi/2} |sinx - cosx|dx = \big[sinx+cosx\big]_{0}^{\pi/4}+ \big[- cosx-sinx\big]_{\pi/4}^{\pi/2}

\int\limits_0^{\pi/2} |sinx - cosx|dx = \bigg(\frac{2}{\sqrt2}-1\bigg)+\bigg(\frac{2}{\sqrt2}-1\bigg)

\int\limits_0^{\pi/2} |sinx - cosx|dx   = 2√2-2

\int\limits_0^{\pi/2} |sinx - cosx|dx   = 2(√2-1)

Problem 6: Prove that:\int\limits_0^{\pi/2}\text{sin2x log tanx dx}=0

Solution:

Let I =\int\limits_0^{\pi/2}\text{sin2x log tanx dx}   ---------(i)

⇒ I =\int\limits_0^{\pi/2}{sin2(\frac{\pi}{2}-x)} \text{log tan}(\frac{\pi}{2}-x) dx

Using property:\int\limits_{0}^{a}f(x)dx=\int\limits_{0}^{a}f(a-x)dx

⇒ I =\int\limits_0^{\pi/2}\text{sin2x log cotx dx}   ---------(ii)

Adding (i) and (ii)

2I =\int\limits_0^{\pi/2}\text{sin2x (log tanx + log cotx)dx}

2I =\int\limits_0^{\pi/2}\text{sin2x (log tanx cotx)dx}

2I =\int\limits_0^{\pi/2}\text{sin2x (log 1)dx}   =0

I = 0

Problem 7: Evaluate:\int\limits_{-\pi}^{\pi}|cosx|dx

Solution:

I =\int\limits_{-\pi}^{\pi}|cosx|dx

|cos x | is an even function

Using property:\int\limits_{-a}^{a}f(x)dx= \begin{cases} 2\int\limits_{0}^{a}f(x)dx & \text{, if $n$ is even} \\ 0 & \text{, if $n$ is odd} \end{cases}

I = 2\int\limits_{0}^{\pi}|cosx|dx

I = 2 {\int\limits_{0}^{\pi/2}|cosx|dx+\int\limits_{\pi/2}^{\pi}|cosx|dx   }

cos x is negative if π/2< x ≤ π

I = 2{\int\limits_{0}^{\pi/2}cosxdx+\int\limits_{\pi/2}^{\pi}-cosxdx   }

I = 2{\big[sinx\big]_0^{\pi/2}-\big[sinx\big]_{\pi/2}^\pi   }

I = 2 + 2 = 4

Problem 8: Evaluate:\int\limits_1^2\frac{\sqrt x}{\sqrt{3-x}\hspace{0.1cm}+\sqrt{x}}dx

Solution:

I =\int\limits_1^2\frac{\sqrt x}{\sqrt{3-x}\hspace{0.1cm}+\sqrt{x}}dx   ----------(i)

Using property\int\limits_{a}^{b}f(x)dx = \int\limits_{a}^{b}f(a + b - x)dx

I =\int\limits_1^2\frac{\sqrt {3-x}}{\sqrt{3-(3-x)}\hspace{0.1cm}+\sqrt{3-x}}dx

I =\int\limits_1^2\frac{\sqrt {3-x}}{\sqrt{x}\hspace{0.1cm}+\sqrt{3-x}}dx   ---------(ii)

Adding (i) and (ii)

2I =\int\limits_1^2\frac{\sqrt {x}+\sqrt {3-x}}{\sqrt{x}\hspace{0.1cm}+\sqrt{3-x}}dx

2I =\int\limits_1^21.dx = \big[x\big]_1^2

2I = 2 - 1

2I = 1

I = 1/2

Problem 9: Show that:\int\limits_0^{\pi/2}f(sin2x)\ sinx\ dx = \sqrt{2}\int\limits_0^{\pi/4}f(cos 2x)\ cosx\ dx

Solution:

I =\int\limits_0^{\pi/2}f(sin2x)\ sinx\ dx

Using property\int\limits_{0}^{a}f(x)dx=\int\limits_{0}^{a}f(a-x)dx

I =\int\limits_0^{\pi/2}f\{sin 2(\frac{\pi}{2}-x)\}\ sin(\frac{\pi}{2}-x)\ dx

I =\int\limits_0^{\pi/2}f\{sin(\pi-2x)\}\ cosx\ dx

I =\int\limits_0^{\pi/2}f(sin2x)\ cosx\ dx

Adding (i) and (ii)

2I =\int\limits_0^{\pi/2}f(sin2x)\ (sinx+cos x)\ dx

Using property\bold{\int\limits_{0}^{2a}f(x)dx= \begin{cases} 2\int\limits_{0}^{a}f(x)dx & , if f(2a - x) = f(x) \\ 0 & , if f(2a - x) = -f(x)\end{cases}}

2I =2\int\limits_0^{\pi/4}f(sin2x)\ (sinx+cos x)\ dx

2I =2\sqrt{2}\int\limits_0^{\pi/4}f(sin 2x)\big( \frac{1}{\sqrt{2}}sinx + \frac{1}{\sqrt{2}}cosx\big)\ dx

2I =2\sqrt{2}\int\limits_0^{\pi/4}f\{sin 2x\}\ sin(x+\frac{\pi}{4})\ dx

Using property\int\limits_{a}^{b}f(x)dx=\int\limits_{a}^{b}f(a+b-x)dx

2I =2\sqrt{2}\int\limits_0^{\pi/4}f\{sin 2(\frac{\pi}{4}-x)\}\ sin(\frac{\pi}{4}-x+\frac{\pi}{4})\ dx

2I =2\sqrt{2}\int\limits_0^{\pi/4}f\{sin (\frac{\pi}{2}-2x)\}\ sin(\frac{\pi}{2}-x)\ dx

2I =2 \sqrt{2}\int\limits_0^{\pi/4}f(cos 2x)\ cosx\ dx

I =\sqrt{2}\int\limits_0^{\pi/4}f(cos 2x)\ cosx\ dx

Problem 10: Evaluate:\int\limits_0^{\pi/2}\frac{x\ sinx\ cosx}{sin^4x+cos^4x}dx

Solution:

I =\int\limits_0^{\pi/2}\frac{x\ sinx\ cosx}{sin^4x+cos^4x}dx

Using property\int\limits_{0}^{a}f(x)dx=\int\limits_{0}^{a}f(a-x)dx

I =\int\limits_0^{\pi/2}\frac{(\frac{\pi}{2}-x)\ sin(\frac{\pi}{2}-x)\ cos(\frac{\pi}{2}-x)}{sin^4(\frac{\pi}{2}-x)+cos^4(\frac{\pi}{2}-x)}dx

I =\int\limits_0^{\pi/2}\frac{(\frac{\pi}{2}-x)\ cosx\ sinx }{cos^4x+sin^4x}dx

Adding (i) and (ii)

2I =\frac{\pi}{2}\int\limits_0^{\pi/2}\frac{sinx\ cosx}{sin^4x+cos^4x}dx

Let t = sin2x ⇒ dt = 2sinx cosx dx

Limits: x= 0, t = 0 and x = π/2, t =1

2I =\frac{\pi}{4}\int\limits_0^{\pi/2}\frac{1}{(1-t)^2+t^2}dt

2I =\frac{\pi}{8}\int\limits_0^{\pi/2}\frac{1}{\big(t-\frac{1}{2}\big)^2+\big(\frac{1}{2}\big)^2}dt

2I =\frac{\pi}{8}\times2\big[tan^{-1}(2t-1)\big]^1_0

I =\frac{\pi}{8}\big(\frac{\pi}{4}+\frac{\pi}{4}\big)

I =\frac{{\pi}^2}{16}


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