INTEGRALS 5

Indefinite Integrals INTEGRALS

The notation ∫ f(x) dx is traditionally used for an antiderivative of f and is called an indefinite integral. Thus, ∫ f(x) dx = F(x) means F’(x) = f(x) INDEFINITE INTEGRAL

INDEFINITE INTEGRALS For example, we can write  Thus, we can regard an indefinite integral as representing an entire family of functions (one antiderivative for each value of the constant C).

INDEFINITE INTEGRALS Any formula can be verified by differentiating the function on the right side and obtaining the integrand. For instance,

TABLE OF INDEFINITE INTEGRALS Table 1

INDEFINITE INTEGRALS Thus, we write with the understanding that it is valid on the interval (0, ∞ ) or on the interval (- ∞, 0).

INDEFINITE INTEGRALS This is true despite the fact that the general antiderivative of the function f(x) = 1/x 2, x ≠ 0, is:

INDEFINITE INTEGRALS Find the general indefinite integral ∫ (10x 4 – 2 sec 2 x) dx  Using our convention and Table 1, we have: ∫(10x 4 – 2 sec 2 x) dx = 10 ∫ x 4 dx – 2 ∫ sec 2 x dx = 10(x 5 /5) – 2 tan x + C = 2x 5 – 2 tan x + C  You should check this answer by differentiating it. Example 1

INDEFINITE INTEGRALS Evaluate  This indefinite integral isn’t immediately apparent in Table 1.  So, we use trigonometric identities to rewrite the function before integrating: Example 2

The Substitution Rule In this section, we will learn: To substitute a new variable in place of an existing expression in a function, making integration easier. INTEGRALS

Our antidifferentiation formulas don’t tell us how to evaluate integrals such as Equation 1 INTRODUCTION

To find this integral, we use the problem- solving strategy of introducing something extra.  The ‘something extra’ is a new variable.  We change from the variable x to a new variable u. INTRODUCTION

Suppose we let u = 1 + x 2 be the quantity under the root sign in the integral below,  Then, the differential of u is du = 2x dx. INTRODUCTION

Notice that, if the dx in the notation for an integral were to be interpreted as a differential, then the differential 2x dx would occur in INTRODUCTION

So, formally, without justifying our calculation, we could write: Equation 2 INTRODUCTION

However, now we can check that we have the correct answer by using the Chain Rule to differentiate INTRODUCTION

In general, this method works whenever we have an integral that we can write in the form ∫ f(g(x))g’(x) dx INTRODUCTION

Observe that, if F’ = f, then ∫ F’(g(x))g’(x) dx = F(g(x)) + C because, by the Chain Rule, Equation 3 INTRODUCTION

If we make the ‘change of variable’ or ‘substitution’ u = g(x), we have: INTRODUCTION

Writing F’ = f, we get: ∫ f(g(x))g’(x) dx = ∫ f(u) du  Thus, we have proved the following rule. INTRODUCTION

SUBSTITUTION RULE If u = g(x) is a differentiable function whose range is an interval I and f is continuous on I, then ∫ f(g(x))g’(x) dx = ∫ f(u) du Equation 4

SUBSTITUTION RULE Notice that the Substitution Rule for integration was proved using the Chain Rule for differentiation. Notice also that, if u = g(x), then du = g’(x) dx.  So, a way to remember the Substitution Rule is to think of dx and du in Equation 4 as differentials.

SUBSTITUTION RULE Thus, the Substitution Rule says: It is permissible to operate with dx and du after integral signs as if they were differentials.

SUBSTITUTION RULE Find ∫ x 3 cos(x 4 + 2) dx  We make the substitution u = x  This is because its differential is du = 4x 3 dx, which, apart from the constant factor 4, occurs in the integral. Example 1

SUBSTITUTION RULE Thus, using x 3 dx = du/4 and the Substitution Rule, we have:  Notice that, at the final stage, we had to return to the original variable x. Example 1

SUBSTITUTION RULE The idea behind the Substitution Rule is to replace a relatively complicated integral by a simpler integral.  This is accomplished by changing from the original variable x to a new variable u that is a function of x.  Thus, in Example 1, we replaced the integral ∫ x 3 cos(x 4 + 2) dx by the simpler integral ¼ ∫ cos u du.

SUBSTITUTION RULE The main challenge in using the rule is to think of an appropriate substitution.  You should try to choose u to be some function in the integrand whose differential also occurs—except for a constant factor.  This was the case in Example 1.

SUBSTITUTION RULE If that is not possible, try choosing u to be some complicated part of the integrand— perhaps the inner function in a composite function.

Finding the right substitution is a bit of an art.  It’s not unusual to guess wrong.  If your first guess doesn’t work, try another substitution. SUBSTITUTION RULE

Evaluate  Let u = 2x + 1.  Then, du = 2 dx.  So, dx = du/2. E. g. 2—Solution 1

 Thus, the rule gives: SUBSTITUTION RULE E. g. 2—Solution 1

SUBSTITUTION RULE Find  Let u = 1 – 4x 2.  Then, du = -8x dx.  So, x dx = -1/8 du and Example 3

SUBSTITUTION RULE Calculate ∫ cos 5x dx  If we let u = 5x, then du = 5 dx.  So, dx = 1/5 du.  Therefore, Example 4