Calculus Examples

$\int \sin (4 x) \cos (4 x) d x$

Step 1

Let $u_{2} = \sin (4 x)$ . Then $d u_{2} = 4 \cos (4 x) d x$ , so $\frac{1}{4} d u_{2} = \cos (4 x) d x$ . Rewrite using $u_{2}$ and $d$ $u_{2}$ .

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Step 1.1

Let $u_{2} = \sin (4 x)$ . Find $\frac{d u_{2}}{d x}$ .

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Step 1.1.1

Differentiate $\sin (4 x)$ .

$\frac{d}{d x} [\sin (4 x)]$

Step 1.1.2

Differentiate using the chain rule, which states that $\frac{d}{d x} [f (g (x))]$ is $f' (g (x)) g' (x)$ where $f (x) = \sin (x)$ and $g (x) = 4 x$ .

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Step 1.1.2.1

To apply the Chain Rule, set $u_{1}$ as $4 x$ .

$\frac{d}{d u_{1}} [\sin (u_{1})] \frac{d}{d x} [4 x]$

Step 1.1.2.2

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

$\cos (u_{1}) \frac{d}{d x} [4 x]$

Step 1.1.2.3

Replace all occurrences of $u_{1}$ with $4 x$ .

$\cos (4 x) \frac{d}{d x} [4 x]$

Step 1.1.3

Differentiate.

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Step 1.1.3.1

Since $4$ is constant with respect to $x$ , the derivative of $4 x$ with respect to $x$ is $4 \frac{d}{d x} [x]$ .

$\cos (4 x) (4 \frac{d}{d x} [x])$

Step 1.1.3.2

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

$\cos (4 x) (4 \cdot 1)$

Step 1.1.3.3

Simplify the expression.

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Step 1.1.3.3.1

Multiply $4$ by $1$ .

$\cos (4 x) \cdot 4$

Step 1.1.3.3.2

Move $4$ to the left of $\cos (4 x)$ .

$4 \cos (4 x)$

Step 1.2

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

$\int u_{2} \frac{1}{4} d u_{2}$

Step 2

Combine $u_{2}$ and $\frac{1}{4}$ .

$\int \frac{u_{2}}{4} d u_{2}$

Step 3

Since $\frac{1}{4}$ is constant with respect to $u_{2}$ , move $\frac{1}{4}$ out of the integral.

$\frac{1}{4} \int u_{2} d u_{2}$

Step 4

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

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

Step 5

Simplify.

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Step 5.1

Rewrite $\frac{1}{4} (\frac{1}{2} {u_{2}}^{2} + C)$ as $\frac{1}{4} \cdot \frac{1}{2} {u_{2}}^{2} + C$ .

$\frac{1}{4} \cdot \frac{1}{2} {u_{2}}^{2} + C$

Step 5.2

Rewrite $\frac{1}{4} \cdot \frac{1}{2} {u_{2}}^{2} + C$ as $\frac{1}{8} {u_{2}}^{2} + C$ .

$\frac{1}{8} {u_{2}}^{2} + C$

Step 6

Replace all occurrences of $u_{2}$ with $\sin (4 x)$ .

$\frac{1}{8} \sin^{2} (4 x) + C$