Calculus Examples

$\int 8 \cos^{3} (2 x) d x$

Step 1

Since $8$ is constant with respect to $x$ , move $8$ out of the integral.

$8 \int \cos^{3} (2 x) d x$

Step 2

Let $u_{1} = 2 x$ . Then $d u_{1} = 2 d x$ , so $\frac{1}{2} d u_{1} = d x$ . Rewrite using $u_{1}$ and $d$ $u_{1}$ .

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Step 2.1

Let $u_{1} = 2 x$ . Find $\frac{d u_{1}}{d x}$ .

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Step 2.1.1

Differentiate $2 x$ .

$\frac{d}{d x} [2 x]$

Step 2.1.2

Since $2$ is constant with respect to $x$ , the derivative of $2 x$ with respect to $x$ is $2 \frac{d}{d x} [x]$ .

$2 \frac{d}{d x} [x]$

Step 2.1.3

Differentiate using the Power Rule which states that $\frac{d}{d x} [x^{n}]$ is $n x^{n - 1}$ where $n = 1$ .

$2 \cdot 1$

Step 2.1.4

Multiply $2$ by $1$ .

$2$

Step 2.2

Rewrite the problem using $u_{1}$ and $d u_{1}$ .

$8 \int \cos^{3} (u_{1}) \frac{1}{2} d u_{1}$

Step 3

Combine $\cos^{3} (u_{1})$ and $\frac{1}{2}$ .

$8 \int \frac{\cos^{3} (u_{1})}{2} d u_{1}$

Step 4

Since $\frac{1}{2}$ is constant with respect to $u_{1}$ , move $\frac{1}{2}$ out of the integral.

$8 (\frac{1}{2} \int \cos^{3} (u_{1}) d u_{1})$

Step 5

Simplify.

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Step 5.1

Combine $\frac{1}{2}$ and $8$ .

$\frac{8}{2} \int \cos^{3} (u_{1}) d u_{1}$

Step 5.2

Cancel the common factor of $8$ and $2$ .

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Step 5.2.1

Factor $2$ out of $8$ .

$\frac{2 \cdot 4}{2} \int \cos^{3} (u_{1}) d u_{1}$

Step 5.2.2

Cancel the common factors.

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Step 5.2.2.1

Factor $2$ out of $2$ .

$\frac{2 \cdot 4}{2 (1)} \int \cos^{3} (u_{1}) d u_{1}$

Step 5.2.2.2

Cancel the common factor.

$\frac{2 \cdot 4}{2 \cdot 1} \int \cos^{3} (u_{1}) d u_{1}$

Step 5.2.2.3

Rewrite the expression.

$\frac{4}{1} \int \cos^{3} (u_{1}) d u_{1}$

Step 5.2.2.4

Divide $4$ by $1$ .

$4 \int \cos^{3} (u_{1}) d u_{1}$

Step 6

Factor out $\cos^{2} (u_{1})$ .

$4 \int \cos^{2} (u_{1}) \cos (u_{1}) d u_{1}$

Step 7

Using the Pythagorean Identity, rewrite $\cos^{2} (u_{1})$ as $1 - \sin^{2} (u_{1})$ .

$4 \int (1 - \sin^{2} (u_{1})) \cos (u_{1}) d u_{1}$

Step 8

Let $u_{2} = \sin (u_{1})$ . Then $d u_{2} = \cos (u_{1}) d u_{1}$ , so $\frac{1}{\cos (u_{1})} d u_{2} = d u_{1}$ . Rewrite using $u_{2}$ and $d$ $u_{2}$ .

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Step 8.1

Let $u_{2} = \sin (u_{1})$ . Find $\frac{d u_{2}}{d u_{1}}$ .

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Step 8.1.1

Differentiate $\sin (u_{1})$ .

$\frac{d}{d u_{1}} [\sin (u_{1})]$

Step 8.1.2

The derivative of $\sin (u_{1})$ with respect to $u_{1}$ is $\cos (u_{1})$ .

$\cos (u_{1})$

Step 8.2

Rewrite the problem using $u_{2}$ and $d u_{2}$ .

$4 \int 1 - {u_{2}}^{2} d u_{2}$

Step 9

Split the single integral into multiple integrals.

$4 (\int d u_{2} + \int - {u_{2}}^{2} d u_{2})$

Step 10

Apply the constant rule.

$4 (u_{2} + C + \int - {u_{2}}^{2} d u_{2})$

Step 11

Since $- 1$ is constant with respect to $u_{2}$ , move $- 1$ out of the integral.

$4 (u_{2} + C - \int {u_{2}}^{2} d u_{2})$

Step 12

By the Power Rule, the integral of ${u_{2}}^{2}$ with respect to $u_{2}$ is $\frac{1}{3} {u_{2}}^{3}$ .

$4 (u_{2} + C - (\frac{1}{3} {u_{2}}^{3} + C))$

Step 13

Simplify.

$4 (u_{2} - \frac{1}{3} {u_{2}}^{3}) + C$

Step 14

Substitute back in for each integration substitution variable.

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Step 14.1

Replace all occurrences of $u_{2}$ with $\sin (u_{1})$ .

$4 (\sin (u_{1}) - \frac{1}{3} \sin^{3} (u_{1})) + C$

Step 14.2

Replace all occurrences of $u_{1}$ with $2 x$ .

$4 (\sin (2 x) - \frac{1}{3} \sin^{3} (2 x)) + C$