Calculus Examples

$f (x) = x \cos (x)$

Step 1

Find the first derivative.

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Step 1.1

Differentiate using the Product Rule which states that $\frac{d}{d x} [f (x) g (x)]$ is $f (x) \frac{d}{d x} [g (x)] + g (x) \frac{d}{d x} [f (x)]$ where $f (x) = x$ and $g (x) = \cos (x)$ .

$f' (x) = x \frac{d}{d x} (\cos (x)) + \cos (x) \frac{d}{d x} (x)$

Step 1.2

The derivative of $\cos (x)$ with respect to $x$ is $- \sin (x)$ .

$f' (x) = x (- \sin (x)) + \cos (x) \frac{d}{d x} (x)$

Step 1.3

Differentiate using the Power Rule.

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Step 1.3.1

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

$f' (x) = x (- \sin (x)) + \cos (x) \cdot 1$

Step 1.3.2

Simplify the expression.

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Step 1.3.2.1

Multiply $\cos (x)$ by $1$ .

$f' (x) = x (- \sin (x)) + \cos (x)$

Step 1.3.2.2

Reorder terms.

$f' (x) = - x \sin (x) + \cos (x)$

Step 2

Find the second derivative.

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Step 2.1

By the Sum Rule, the derivative of $- x \sin (x) + \cos (x)$ with respect to $x$ is $\frac{d}{d x} [- x \sin (x)] + \frac{d}{d x} [\cos (x)]$ .

$f'' (x) = \frac{d}{d x} (- x \sin (x)) + \frac{d}{d x} (\cos (x))$

Step 2.2

Evaluate $\frac{d}{d x} [- x \sin (x)]$ .

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Step 2.2.1

Since $- 1$ is constant with respect to $x$ , the derivative of $- x \sin (x)$ with respect to $x$ is $- \frac{d}{d x} [x \sin (x)]$ .

$f'' (x) = - \frac{d}{d x} (x \sin (x)) + \frac{d}{d x} (\cos (x))$

Step 2.2.2

Differentiate using the Product Rule which states that $\frac{d}{d x} [f (x) g (x)]$ is $f (x) \frac{d}{d x} [g (x)] + g (x) \frac{d}{d x} [f (x)]$ where $f (x) = x$ and $g (x) = \sin (x)$ .

$f'' (x) = - (x \frac{d}{d x} (\sin (x)) + \sin (x) \frac{d}{d x} (x)) + \frac{d}{d x} (\cos (x))$

Step 2.2.3

The derivative of $\sin (x)$ with respect to $x$ is $\cos (x)$ .

$f'' (x) = - (x \cos (x) + \sin (x) \frac{d}{d x} (x)) + \frac{d}{d x} (\cos (x))$

Step 2.2.4

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

$f'' (x) = - (x \cos (x) + \sin (x) \cdot 1) + \frac{d}{d x} (\cos (x))$

Step 2.2.5

Multiply $\sin (x)$ by $1$ .

$f'' (x) = - (x \cos (x) + \sin (x)) + \frac{d}{d x} (\cos (x))$

Step 2.3

The derivative of $\cos (x)$ with respect to $x$ is $- \sin (x)$ .

$f'' (x) = - (x \cos (x) + \sin (x)) - \sin (x)$

Step 2.4

Simplify.

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Step 2.4.1

Apply the distributive property.

$f'' (x) = - (x \cos (x)) - \sin (x) - \sin (x)$

Step 2.4.2

Subtract $\sin (x)$ from $- \sin (x)$ .

$f'' (x) = - x \cos (x) - 2 \sin (x)$

Step 3

Find the third derivative.

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Step 3.1

By the Sum Rule, the derivative of $- x \cos (x) - 2 \sin (x)$ with respect to $x$ is $\frac{d}{d x} [- x \cos (x)] + \frac{d}{d x} [- 2 \sin (x)]$ .

$f''' (x) = \frac{d}{d x} (- x \cos (x)) + \frac{d}{d x} (- 2 \sin (x))$

Step 3.2

Evaluate $\frac{d}{d x} [- x \cos (x)]$ .

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Step 3.2.1

Since $- 1$ is constant with respect to $x$ , the derivative of $- x \cos (x)$ with respect to $x$ is $- \frac{d}{d x} [x \cos (x)]$ .

$f''' (x) = - \frac{d}{d x} (x \cos (x)) + \frac{d}{d x} (- 2 \sin (x))$

Step 3.2.2

Differentiate using the Product Rule which states that $\frac{d}{d x} [f (x) g (x)]$ is $f (x) \frac{d}{d x} [g (x)] + g (x) \frac{d}{d x} [f (x)]$ where $f (x) = x$ and $g (x) = \cos (x)$ .

$f''' (x) = - (x \frac{d}{d x} (\cos (x)) + \cos (x) \frac{d}{d x} (x)) + \frac{d}{d x} (- 2 \sin (x))$

Step 3.2.3

The derivative of $\cos (x)$ with respect to $x$ is $- \sin (x)$ .

$f''' (x) = - (x (- \sin (x)) + \cos (x) \frac{d}{d x} (x)) + \frac{d}{d x} (- 2 \sin (x))$

Step 3.2.4

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

$f''' (x) = - (x (- \sin (x)) + \cos (x) \cdot 1) + \frac{d}{d x} (- 2 \sin (x))$

Step 3.2.5

Multiply $\cos (x)$ by $1$ .

$f''' (x) = - (x (- \sin (x)) + \cos (x)) + \frac{d}{d x} (- 2 \sin (x))$

Step 3.3

Evaluate $\frac{d}{d x} [- 2 \sin (x)]$ .

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Step 3.3.1

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

$f''' (x) = - (x (- \sin (x)) + \cos (x)) - 2 \frac{d}{d x} \sin (x)$

Step 3.3.2

The derivative of $\sin (x)$ with respect to $x$ is $\cos (x)$ .

$f''' (x) = - (x (- \sin (x)) + \cos (x)) - 2 \cos (x)$

Step 3.4

Simplify.

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Step 3.4.1

Apply the distributive property.

$f''' (x) = - (x (- \sin (x))) - \cos (x) - 2 \cos (x)$

Step 3.4.2

Combine terms.

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Step 3.4.2.1

Multiply $- 1$ by $- 1$ .

$f''' (x) = 1 (x (\sin (x))) - \cos (x) - 2 \cos (x)$

Step 3.4.2.2

Multiply $x$ by $1$ .

$f''' (x) = x \sin (x) - \cos (x) - 2 \cos (x)$

Step 3.4.2.3

Subtract $2 \cos (x)$ from $- \cos (x)$ .

$f''' (x) = x \sin (x) - 3 \cos (x)$

Step 4

Find the fourth derivative.

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Step 4.1

By the Sum Rule, the derivative of $x \sin (x) - 3 \cos (x)$ with respect to $x$ is $\frac{d}{d x} [x \sin (x)] + \frac{d}{d x} [- 3 \cos (x)]$ .

$f^{4} (x) = \frac{d}{d x} (x \sin (x)) + \frac{d}{d x} (- 3 \cos (x))$

Step 4.2

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

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Step 4.2.1

Differentiate using the Product Rule which states that $\frac{d}{d x} [f (x) g (x)]$ is $f (x) \frac{d}{d x} [g (x)] + g (x) \frac{d}{d x} [f (x)]$ where $f (x) = x$ and $g (x) = \sin (x)$ .

$f^{4} (x) = x \frac{d}{d x} (\sin (x)) + \sin (x) \frac{d}{d x} (x) + \frac{d}{d x} (- 3 \cos (x))$

Step 4.2.2

The derivative of $\sin (x)$ with respect to $x$ is $\cos (x)$ .

$f^{4} (x) = x \cos (x) + \sin (x) \frac{d}{d x} (x) + \frac{d}{d x} (- 3 \cos (x))$

Step 4.2.3

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

$f^{4} (x) = x \cos (x) + \sin (x) \cdot 1 + \frac{d}{d x} (- 3 \cos (x))$

Step 4.2.4

Multiply $\sin (x)$ by $1$ .

$f^{4} (x) = x \cos (x) + \sin (x) + \frac{d}{d x} (- 3 \cos (x))$

Step 4.3

Evaluate $\frac{d}{d x} [- 3 \cos (x)]$ .

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Step 4.3.1

Since $- 3$ is constant with respect to $x$ , the derivative of $- 3 \cos (x)$ with respect to $x$ is $- 3 \frac{d}{d x} [\cos (x)]$ .

$f^{4} (x) = x \cos (x) + \sin (x) - 3 \frac{d}{d x} \cos (x)$

Step 4.3.2

The derivative of $\cos (x)$ with respect to $x$ is $- \sin (x)$ .

$f^{4} (x) = x \cos (x) + \sin (x) - 3 (- \sin (x))$

Step 4.3.3

Multiply $- 1$ by $- 3$ .

$f^{4} (x) = x \cos (x) + \sin (x) + 3 \sin (x)$

Step 4.4

Add $\sin (x)$ and $3 \sin (x)$ .

$f^{4} (x) = x \cos (x) + 4 \sin (x)$

Step 5

The fourth derivative of $f (x)$ with respect to $x$ is $x \cos (x) + 4 \sin (x)$ .

$x \cos (x) + 4 \sin (x)$