Integration is a fundamental concept in calculus that involves finding the antiderivative of a given function. While some integrals can be easy to solve, others can be quite challenging. Integration is a fundamental concept in calculus that involves finding the antiderivative of a given function. While some integrals can be easy to solve, others can be quite challenging. **Integration by substitution**, also known as u-substitution, is a powerful technique that allows us to simplify **complex integrals** by reversing the chain rule of differentiation. Let’s understand integration by substitution and explore how it is similar to the chain rule of derivatives.

## Understanding Integration by Substitution

Integration by substitution is an integral technique which is used to simplify integration of functions. It is also known as u-substitution because it involves substituting a new variable in place of the existing variable in an integral.

This new variable is chosen in such a way that it simplifies the integral expression. The goal is to transform the integral into a form that can be easily integrated using basic integration techniques.

### Trigonometric Substitution

There is another type of integration by substitution method which is known as **trigonometric substitution** or simply integration by substitution. In this method, we use specific substitutions to solve integrals.

The trigonometric substitution helps to solve integrals by using trigonometric functions. Some important substitutions are:

To solve integrals by using this method, **trigonometric substitution calculator** can provide a stepwise solution.

### The Chain Rule and Its Inverse

The chain rule is a fundamental concept in calculus. It enables us to differentiate composite functions. It states that if we have a composite function f(g(x)), the derivative can be calculated as the product of the derivative of the outer function (f'(g(x))) and the derivative of the inner function (g'(x)).

Mathematically, the chain rule for a function f(g(x)) is expressed as: dy/dx = dy/du x du/dx

Where, u=g(x) and y=f(u). You can also calculate the derivative of a composite function by using a **chain rule calculator**.

Integration by substitution is essentially the reverse of this process. We replace the integrand with a variable u in order to write it in simple form. It helps us to solve the integration easily.

## How to Solve Integration by Substitution?

Integration by substitution follows a systematic approach that involves the following steps:

- Identify a suitable substitution variable ‘u’ according to the given integrand.
- Find the derivative du/dx of the substitution variable ‘u’ with respect to the variable of integration ‘x’.
- Now rearrange the integral, replacing the original variable and its derivative with the substitution variable and its differential.
- Substitute the new variable ‘u’ and its derivative into the integral.
- Simplify the integral and evaluate it in terms of ‘u.’
- Replace ‘u’ with the original variable to obtain the final result.

### Example of Integration by Substitution

Let’s understand how to solve integration by substitution in the following example. Consider the integral:

I=∫ (3x^2 + 2x) √(x^3 + x^2 + x + 1) dx.

We can use integration by substitution to simplify this integral.

Let u = x^3 + x^2 + x + 1.

Differentiating u with respect to x, we get

du = (3x^2 + 2x) dx

Substitute u and du into the integral expression. We have:

I = ∫ √u du.

The new integral is ∫ √u du, which can be evaluated using basic integration techniques. After integrating, we obtain:

∫ √u du=(2/3)u^(3/2) + C

where C is the constant of integration.

Finally, express the result in terms of the original variable, x.

Substitute u back in terms of x:(2/3)(x^3 + x^2 + x + 1)^(3/2) + C.

Using the u-substitution calculator online can help you to solve integrals easily.

### Example of Trigonometric Substitution

Consider the integral:

I =∫ x² √(1 – x²) dx

To simplify this integral, we can use the trigonometric substitution x = sinθ.

Differentiating x = sinθ with respect to θ gives,

dx/dθ = cosθ.

Rearranging this equation, we have dx = cosθ dθ.

Substituting the original expression and the differential into the integral, we have:

∫ sin²θ √(1 – sin²θ) cosθ dθ

Simplifying the expression, we have:

∫ sin²θ cos²θ dθ

Applying a trigonometric identity, sin²θ cos²θ = (1/4)sin²(2θ), we can rewrite the integral as:

(1/4) ∫ sin²(2θ) dθ

Using a standard integral formula, we can evaluate this integral:

(1/4) [(θ/2) – (1/4)sin(4θ)] + C

Substituting back θ = sin⁻¹(x), we obtain the final result:

(1/8) [sin⁻¹(x) – x√(1 – x²)(1 – 2x²)] + C

## Applications of Integration by Substitution

Integration by substitution has applications in various areas of mathematics and physics. Some most important applications are:

- Solving integrals involving algebraic, trigonometric, or exponential functions.
- Evaluating definite integrals to find areas, volumes, or solutions to differential equations.
- Simplifying complex integrals by transforming them into more manageable forms.

#### Conclusion

**Integration by substitution** is a fundamental technique of integration in calculus which is also known as u-substitution. It is because we use a variable u to write the integrand in a simple form. This variable is known as the variable of substitution. Choosing an exact variable of substitution helps us to simplify complex integrals easily. It has a wide range of **applications in calculus** as it helps to solve many PDEs and ODEs.