Precalculus Examples

$\cos (4 a)$

Step 1

Factor $2$ out of $4 a$ .

$\cos (2 (2 a))$

Step 2

Simplify each term.

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Step 2.1

Use the double-angle identity to transform $\cos (2 x)$ to $2 \cos^{2} (x) - 1$ .

$2 {(2 \cos^{2} (a) - 1)}^{2} - 1$

Step 2.2

Rewrite ${(2 \cos^{2} (a) - 1)}^{2}$ as $(2 \cos^{2} (a) - 1) (2 \cos^{2} (a) - 1)$ .

$2 ((2 \cos^{2} (a) - 1) (2 \cos^{2} (a) - 1)) - 1$

Step 2.3

Expand $(2 \cos^{2} (a) - 1) (2 \cos^{2} (a) - 1)$ using the FOIL Method.

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Step 2.3.1

Apply the distributive property.

$2 (2 \cos^{2} (a) (2 \cos^{2} (a) - 1) - 1 (2 \cos^{2} (a) - 1)) - 1$

Step 2.3.2

Apply the distributive property.

$2 (2 \cos^{2} (a) (2 \cos^{2} (a)) + 2 \cos^{2} (a) \cdot - 1 - 1 (2 \cos^{2} (a) - 1)) - 1$

Step 2.3.3

Apply the distributive property.

$2 (2 \cos^{2} (a) (2 \cos^{2} (a)) + 2 \cos^{2} (a) \cdot - 1 - 1 (2 \cos^{2} (a)) - 1 \cdot - 1) - 1$

Step 2.4

Simplify and combine like terms.

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Step 2.4.1

Simplify each term.

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Step 2.4.1.1

Rewrite using the commutative property of multiplication.

$2 (2 \cdot 2 \cos^{2} (a) \cos^{2} (a) + 2 \cos^{2} (a) \cdot - 1 - 1 (2 \cos^{2} (a)) - 1 \cdot - 1) - 1$

Step 2.4.1.2

Multiply $\cos^{2} (a)$ by $\cos^{2} (a)$ by adding the exponents.

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Step 2.4.1.2.1

Move $\cos^{2} (a)$ .

$2 (2 \cdot 2 (\cos^{2} (a) \cos^{2} (a)) + 2 \cos^{2} (a) \cdot - 1 - 1 (2 \cos^{2} (a)) - 1 \cdot - 1) - 1$

Step 2.4.1.2.2

Use the power rule $a^{m} a^{n} = a^{m + n}$ to combine exponents.

$2 (2 \cdot 2 {\cos (a)}^{2 + 2} + 2 \cos^{2} (a) \cdot - 1 - 1 (2 \cos^{2} (a)) - 1 \cdot - 1) - 1$

Step 2.4.1.2.3

Add $2$ and $2$ .

$2 (2 \cdot 2 \cos^{4} (a) + 2 \cos^{2} (a) \cdot - 1 - 1 (2 \cos^{2} (a)) - 1 \cdot - 1) - 1$

Step 2.4.1.3

Multiply $2$ by $2$ .

$2 (4 \cos^{4} (a) + 2 \cos^{2} (a) \cdot - 1 - 1 (2 \cos^{2} (a)) - 1 \cdot - 1) - 1$

Step 2.4.1.4

Multiply $- 1$ by $2$ .

$2 (4 \cos^{4} (a) - 2 \cos^{2} (a) - 1 (2 \cos^{2} (a)) - 1 \cdot - 1) - 1$

Step 2.4.1.5

Multiply $2$ by $- 1$ .

$2 (4 \cos^{4} (a) - 2 \cos^{2} (a) - 2 \cos^{2} (a) - 1 \cdot - 1) - 1$

Step 2.4.1.6

Multiply $- 1$ by $- 1$ .

$2 (4 \cos^{4} (a) - 2 \cos^{2} (a) - 2 \cos^{2} (a) + 1) - 1$

Step 2.4.2

Subtract $2 \cos^{2} (a)$ from $- 2 \cos^{2} (a)$ .

$2 (4 \cos^{4} (a) - 4 \cos^{2} (a) + 1) - 1$

Step 2.5

Apply the distributive property.

$2 (4 \cos^{4} (a)) + 2 (- 4 \cos^{2} (a)) + 2 \cdot 1 - 1$

Step 2.6

Simplify.

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Step 2.6.1

Multiply $4$ by $2$ .

$8 \cos^{4} (a) + 2 (- 4 \cos^{2} (a)) + 2 \cdot 1 - 1$

Step 2.6.2

Multiply $- 4$ by $2$ .

$8 \cos^{4} (a) - 8 \cos^{2} (a) + 2 \cdot 1 - 1$

Step 2.6.3

Multiply $2$ by $1$ .

$8 \cos^{4} (a) - 8 \cos^{2} (a) + 2 - 1$

Step 3

Subtract $1$ from $2$ .

$8 \cos^{4} (a) - 8 \cos^{2} (a) + 1$