Calculus Examples

$y = 2 x \cos (x)$

Step 1

Find the first derivative.

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

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

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 $g (x) = \cos (x)$ .

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

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

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

Differentiate using the Power Rule.

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

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

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

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

Simplify.

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Apply the distributive property.

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

Multiply $- 1$ by $2$ .

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

Step 2

Find the second derivative.

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

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

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

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

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

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 $g (x) = \sin (x)$ .

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

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

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

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

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

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

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

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

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

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

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

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

Multiply $- 1$ by $2$ .

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

Simplify.

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Apply the distributive property.

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

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

$f'' (x) = - 2 x \cos (x) - 4 \sin (x)$