 # Evaluating Definite Integrals

Integration and differentiation are the two important process in Calculus. Differentiation is the process of finding the derivative of a function, whereas integration is the reverse process of differentiation. It means that the process of finding the anti-derivative of a function. In integration, the concept behind are functions, limits and integrals. The integrals are generally classified into two different types, namely:

In this article, we are going to discuss the definition of definite integrals, and the process of evaluating the definite integral using different properties.

## What is a Definite Integral?

If the upper limit and the lower limit of the independent variable of the given function or integrand is specified, its integration is expressed using definite integrals. A definite integral is denoted as:

$F(a) – F(b) = \int\limits_{a}^b f(x)dx$

Here R.H.S. of the equation means integral of f(x) with respect to x.

f(x)is called the integrand.

dx is called the integrating agent.

a is the upper limit of the integral and b is the lower limit of the integral.

## Evaluating Definite Integrals – Properties

Let us now discuss important properties of definite integrals and their proofs.

Property 1: $\int\limits_{a}^b$f(x)dx= $\int\limits_{a}^b$f(t)dt

Let us consider x = t. Therefore dx = dt. Substituting these values in the LHS of the above equation we can prove this property.

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

According to second fundamental theorem of Calculus, if f(x) is a continuous function defined on the closed interval [a, b] and F(x) denotes the anti-derivative of f(x), then

$\int\limits_{a}^b$f(x)dx= [F(x)${]}_{a}^{b}$= F(b)-F(a)

Therefore

$\int\limits_{a}^b$f(x)dx= F(b)-F(a) = – [F(a)-F(b)= -$\int\limits_{b}^a$f(x)dx

Property 3: $\int\limits_{a}^b$dx= $\int\limits_{a}^c$f(x)dx + $\int\limits_{c}^b$f(x)dx

From the second theorem of Calculus,

$\int\limits_{a}^b$f(x)dx= [F(x)${]}_{a}^{b}$= F(b)-F(a)

Therefore,

$\int\limits_{a}^b$f(x)dx= F(b)-F(a)——-(1)

$\int\limits_{a}^c$f(x)dx= F(c)-F(a)——-(2)

$\int\limits_{c}^b$f(x)dx= F(b)-F(c)——-(3)

Adding equations 2 and 3 we have,

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

Property 4:  $\int\limits_{a}^b$f(x)dx= $\int\limits_{a}^b$f(a+b-x)dx

Let us assume that t = a + b – x. Therefore dt = -dx. At x = a, t = b and at x = b, t =a.

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

From the second property of definite integrals:
$\int\limits_{a}^b$f(x)dx=-$\int\limits_{b}^a$f(x)dx

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

Following the first property of definite integrals:
$\int\limits_{a}^b$f(x)dx= $\int\limits_{a}^b$f(a+b-x)dx

Property 5: $\int\limits_{0}^a$f(x)dx=$\int\limits_{0}^a$f(a-x)dx

This property is a special case of fourth property of integrals as discussed above.
Let us assume that t = a – x. Therefore dt = -dx. At x = 0, t = a and at x = a, t =0.

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

From the second property of definite integrals:
$\int\limits_{0}^a$f(x)dx=-$\int\limits_{0}^a$f(x)dx

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

Following the first property of definite integrals:
$\int\limits_{0}^a$f(x)dx=$\int\limits_{0}^a$f(a-x)dx

Property 6: $\int\limits_{0}^{2a}$f(x)dx=$\int\limits_{0}^a$f(x)f(x)dx+$\int\limits_{0}^a$f(2a-x)dx

From the second property of definite integrals
$\int\limits_{a}^b$f(x)dx= $\int\limits_{a}^c$f(x)dx+$\int\limits_{c}^b$f(x)dx

Therefore,
$\int\limits_{0}^{2a}$f(x)dx=$\int\limits_{0}^a$f(x)dx+$\int\limits_{a}^{2a}$f(x)dx——–(4)

Let us assume that t = 2a – x. Then dt = – dx. In such a case, when x = 2a, t = 0. Therefore, the second integral can be expressed as:

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

Equation 4 can, therefore, be rewritten as:
$\int\limits_{0}^{2a}$f(x)dx=$\int\limits_{0}^a$f(x)dx+$\int\limits_{0}^a$f(2a-x)dx

Property 7:  $\int\limits_{0}^{2a}$f(x)dx= 2$\int\limits_{0}^a$f(x)dx if f(2a-x)=f(x) and $\int\limits_{0}^{2a}$f(x)dx=0 if f(2a-x)=-f(x)

Using the sixth property of definite integrals, we have
$\int\limits_{0}^2a$f(x)dx=$\int\limits_{0}^a$f(x)dx+$\int\limits_{0}^a$f(2a-x)dx——-(5)

If in case f (2a-x)=f(x), therefore equation 5 can be rewritten as:
$\int\limits_{0}^{2a}$f(x)dx=$\int\limits_{0}^a$f(x)dx+$\int\limits_{0}^a$f(x)dx

If f(2a – x) = -f(x), then equation 5 can be rewritten as:
$\int\limits_{0}^{2a}$f(x)dx=$\int\limits_{0}^a$f(x)dx -$\int\limits_{0}^a$f(x)dx

Property 8: $\int\limits_{-a}^a$f(x)dx=2$\int\limits_{0}^a$f(x)dx if f(-x)=f(x) and $\int\limits_{-a}^a$f(x)dx=0 if f(-x)= -f(x)

Using third property of definite integrals we have
$\int\limits_{a}^b$f(x)dx=$\int\limits_{a}^c$f(x)dx +$\int\limits_{c}^b$f(x)dx

Therefore,
$\int\limits_{-a}^a$f(x)dx=$\int\limits_{-a}^0$f(x)dx+$\int\limits_{0}^a$f(x)dx——–(6)

Let us assume that t = -x. Then dt = -dx. Thus when x = -a, t = a and x =0, t = 0. Then the first integral in right hand side can be written as:
$\int\limits_{-a}^0$f(x)dx= -$\int\limits_{a}^0$f(-t)dt

Therefore equation 6 can be written as;
$\int\limits_{-a}^a$f(x)dx= -$\int\limits_{a}^0$f(-t)dt + $\int\limits_{0}^a$f(x)dx

$\int\limits_{-a}^a$f(x)dx= $\int\limits_{0}^a$f(-x)dx + $\int\limits_{0}^a$f(x)dx———(7)

In case if f is an even function, i.e., f(-x) = f(x), then equation 7 can be rewritten as:
$\int\limits_{-a}^a$f(x)dx= $\int\limits_{0}^a$f(x)dx+ $\int\limits_{0}^a$f(x)dx= 2$\int\limits_{0}^a$f(x)dx

In case if f is an odd function, i.e., f(-x) = – f(x), then equation 7 can be rewritten as:
$\int\limits_{-a}^a$f(x)dx= $\int\limits_{0}^a$f(x)dx- $\int\limits_{0}^a$f(x)dx= 0

Stay tuned with BYJU’S – The Learning App and download the app to learn all the important Maths-related concepts with ease.