# Calculus/Integration techniques/Recognizing Derivatives and the Substitution Rule

After learning a simple list of antiderivatives, it is time to move on to more complex integrands, which are not at first readily integrable. In these first steps, we notice certain special case integrands which can be easily integrated in a few steps.

## Contents

## Recognizing Derivatives and Reversing Derivative Rules[edit]

If we recognize a function as being the derivative of a function , then we can easily express the antiderivative of :

For example, since

we can conclude that

Similarly, since we know is its own derivative,

The power rule for derivatives can be reversed to give us a way to handle integrals of powers of . Since

,

we can conclude that

or, a little more usefully,

.

## Integration by Substitution[edit]

For many integrals, a substitution can be used to transform the integrand and make possible the finding of an antiderivative. There are a variety of such substitutions, each depending on the form of the integrand.

The objective of Integration by substitution is to substitute the integrand from an expression with variable x to an expression with variable where

**Theory**

We want to transform the Integral from a function of x to a function of u

Starting with

**Steps**

(1) | ie | |||

(2) | ie | |||

(3) | ie | |||

(4) | ie Now equate with | |||

(5) | ie | |||

(6) | ie | |||

(7) | ie We have achieved our desired result |

**Procedure**

- Calculate
- Calculate which is and
**make sure you express the result in terms of the variable u** - Calculate
- Calculate

### Integrating with the derivative present[edit]

If a component of the integrand can be viewed as the derivative of another component of the integrand, a substitution can be made to simplify the integrand.

For example, in the integral

we see that is the derivative of . Letting

we have

or, in order to apply it to the integral,

- .

With this we may write

Note that it was not necessary that we had *exactly* the derivative of in our integrand. It would have been sufficient to have any constant multiple of the derivative.

For instance, to treat the integral

we may let . Then

and so

the right-hand side of which is a factor of our integrand. Thus,

In general, the integral of a power of a function times that function's derivative may be integrated in this way. Since ,

we have

Therefore, | |

There is a similar rule for definite integrals, but we have to change the endpoints.

**Substitution rule for definite integrals**

*u*is differentiable with continuous derivative and that

*f*is continuous on the range of

*u*. Suppose and . Then

### Examples[edit]

Consider the integral

By using the substitution *u* = *x*^{2} + 1, we obtain *du* = 2*x* *dx* and

Note how the lower limit *x* = 0 was transformed into *u* = 0^{2} + 1 = 1 and the upper limit *x* = 2 into *u* = 2^{2} + 1 = 5.

### Proof of the substitution rule[edit]

We will now prove the substitution rule for definite integrals. Let *H* be an anti derivative of *h* so

- .

Suppose we have a differentiable function, such that , and numbers and derived from some given numbers, and .

By the Fundamental Theorem of Calculus, we have

Next we define a function by the rule

Naturally

Then by the Chain rule *F* is differentiable with derivative

Integrating both sides with respect to and using the Fundamental Theorem of Calculus we get

But by the definition of this equals

Hence

which is the substitution rule for definite integrals.

### Exercises[edit]

Evaluate the following using a suitable substitution.