Trigonometry Examples

$\cos (4 x)$

Step 1

A good method to expand $\cos (4 x)$ is by using De Moivre's theorem $(r {(\cos (x) + i \cdot \sin (x))}^{n} = r^{n} (\cos (n x) + i \cdot \sin (n x)))$ . When $r = 1$ , $\cos (n x) + i \cdot \sin (n x) = {(\cos (x) + i \cdot \sin (x))}^{n}$ .

$\cos (n x) + i \cdot \sin (n x) = {(\cos (x) + i \cdot \sin (x))}^{n}$

Step 2

Expand the right hand side of $\cos (n x) + i \cdot \sin (n x) = {(\cos (x) + i \cdot \sin (x))}^{n}$ using the binomial theorem.

Expand: ${(\cos (x) + i \cdot \sin (x))}^{4}$

Step 3

Use the Binomial Theorem.

$\cos^{4} (x) + 4 \cos^{3} (x) (i \sin (x)) + 6 \cos^{2} (x) {(i \sin (x))}^{2} + 4 \cos (x) {(i \sin (x))}^{3} + {(i \sin (x))}^{4}$

Step 4

Simplify terms.

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Step 4.1

Simplify each term.

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Step 4.1.1

Apply the product rule to $i \sin (x)$ .

$\cos^{4} (x) + 4 \cos^{3} (x) i \sin (x) + 6 \cos^{2} (x) (i^{2} \sin^{2} (x)) + 4 \cos (x) {(i \sin (x))}^{3} + {(i \sin (x))}^{4}$

Step 4.1.2

Rewrite using the commutative property of multiplication.

$\cos^{4} (x) + 4 \cos^{3} (x) i \sin (x) + 6 \cdot i^{2} \cos^{2} (x) \sin^{2} (x) + 4 \cos (x) {(i \sin (x))}^{3} + {(i \sin (x))}^{4}$

Step 4.1.3

Rewrite $i^{2}$ as $- 1$ .

$\cos^{4} (x) + 4 \cos^{3} (x) i \sin (x) + 6 \cdot - 1 \cos^{2} (x) \sin^{2} (x) + 4 \cos (x) {(i \sin (x))}^{3} + {(i \sin (x))}^{4}$

Step 4.1.4

Multiply $6$ by $- 1$ .

$\cos^{4} (x) + 4 \cos^{3} (x) i \sin (x) - 6 \cos^{2} (x) \sin^{2} (x) + 4 \cos (x) {(i \sin (x))}^{3} + {(i \sin (x))}^{4}$

Step 4.1.5

Apply the product rule to $i \sin (x)$ .

$\cos^{4} (x) + 4 \cos^{3} (x) i \sin (x) - 6 \cos^{2} (x) \sin^{2} (x) + 4 \cos (x) (i^{3} \sin^{3} (x)) + {(i \sin (x))}^{4}$

Step 4.1.6

Factor out $i^{2}$ .

$\cos^{4} (x) + 4 \cos^{3} (x) i \sin (x) - 6 \cos^{2} (x) \sin^{2} (x) + 4 \cos (x) (i^{2} \cdot i \sin^{3} (x)) + {(i \sin (x))}^{4}$

Step 4.1.7

Rewrite $i^{2}$ as $- 1$ .

$\cos^{4} (x) + 4 \cos^{3} (x) i \sin (x) - 6 \cos^{2} (x) \sin^{2} (x) + 4 \cos (x) (- 1 \cdot i \sin^{3} (x)) + {(i \sin (x))}^{4}$

Step 4.1.8

Rewrite $- 1 i$ as $- i$ .

$\cos^{4} (x) + 4 \cos^{3} (x) i \sin (x) - 6 \cos^{2} (x) \sin^{2} (x) + 4 \cos (x) (- i \sin^{3} (x)) + {(i \sin (x))}^{4}$

Step 4.1.9

Multiply $- 1$ by $4$ .

$\cos^{4} (x) + 4 \cos^{3} (x) i \sin (x) - 6 \cos^{2} (x) \sin^{2} (x) - 4 \cos (x) (i \sin^{3} (x)) + {(i \sin (x))}^{4}$

Step 4.1.10

Apply the product rule to $i \sin (x)$ .

$\cos^{4} (x) + 4 \cos^{3} (x) i \sin (x) - 6 \cos^{2} (x) \sin^{2} (x) - 4 \cos (x) i \sin^{3} (x) + i^{4} \sin^{4} (x)$

Step 4.1.11

Rewrite $i^{4}$ as $1$ .

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Step 4.1.11.1

Rewrite $i^{4}$ as ${(i^{2})}^{2}$ .

$\cos^{4} (x) + 4 \cos^{3} (x) i \sin (x) - 6 \cos^{2} (x) \sin^{2} (x) - 4 \cos (x) i \sin^{3} (x) + {(i^{2})}^{2} \sin^{4} (x)$

Step 4.1.11.2

Rewrite $i^{2}$ as $- 1$ .

$\cos^{4} (x) + 4 \cos^{3} (x) i \sin (x) - 6 \cos^{2} (x) \sin^{2} (x) - 4 \cos (x) i \sin^{3} (x) + {(- 1)}^{2} \sin^{4} (x)$

Step 4.1.11.3

Raise $- 1$ to the power of $2$ .

$\cos^{4} (x) + 4 \cos^{3} (x) i \sin (x) - 6 \cos^{2} (x) \sin^{2} (x) - 4 \cos (x) i \sin^{3} (x) + 1 \sin^{4} (x)$

Step 4.1.12

Multiply $\sin^{4} (x)$ by $1$ .

$\cos^{4} (x) + 4 \cos^{3} (x) i \sin (x) - 6 \cos^{2} (x) \sin^{2} (x) - 4 \cos (x) i \sin^{3} (x) + \sin^{4} (x)$

Step 4.2

Reorder factors in $\cos^{4} (x) + 4 \cos^{3} (x) i \sin (x) - 6 \cos^{2} (x) \sin^{2} (x) - 4 \cos (x) i \sin^{3} (x) + \sin^{4} (x)$ .

$\cos^{4} (x) + 4 i \cos^{3} (x) \sin (x) - 6 \cos^{2} (x) \sin^{2} (x) - 4 i \cos (x) \sin^{3} (x) + \sin^{4} (x)$

Step 5

Take out the expressions with the imaginary part, which are equal to $\cos (4 x)$ . Remove the imaginary number $i$ .

$\cos (4 x) = \cos^{4} (x) - 6 \cos^{2} (x) \sin^{2} (x) + \sin^{4} (x)$