Is cos^3(x)= 3/4 cos(x)+1/4 cos(3x) ?

The answer to whether cos^3(x)= 3/4 cos(x)+1/4 cos(3x) is True

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Popular Trigonometry >

prove cos^3(x)= 3/4 cos(x)+1/4 cos(3x)

Solution

prove

cos^{3} (x) = \frac{3}{4} cos (x) + \frac{1}{4} cos (3 x)

Solution

True

Solution steps

cos^{3} (x) = \frac{3}{4} cos (x) + \frac{1}{4} cos (3 x)

Manipulating right side

\frac{3}{4} cos (x) + \frac{1}{4} cos (3 x)

Rewrite using trig identities

\frac{3}{4} cos (x) + \frac{1}{4} cos (3 x)

Use the following identity:

cos (3 x) = 4 cos^{3} (x) - 3 cos (x)

cos (3 x)

Rewrite using trig identities

cos (3 x)

Rewrite as

= cos (2 x + x)

Use the Angle Sum identity:

cos (s + t) = cos (s) cos (t) - sin (s) sin (t)

= cos (2 x) cos (x) - sin (2 x) sin (x)

Use the Double Angle identity:

sin (2 x) = 2 sin (x) cos (x)

= cos (2 x) cos (x) - 2 sin (x) cos (x) sin (x)

Simplify

cos (2 x) cos (x) - 2 sin (x) cos (x) sin (x) : cos (x) cos (2 x) - 2 sin^{2} (x) cos (x)

cos (2 x) cos (x) - 2 sin (x) cos (x) sin (x)

2 sin (x) cos (x) sin (x) = 2 sin^{2} (x) cos (x)

2 sin (x) cos (x) sin (x)

Apply exponent rule:

a^{b} \cdot a^{c} = a^{b + c}

sin (x) sin (x) = sin^{1 + 1} (x)

= 2 cos (x) sin^{1 + 1} (x)

Add the numbers:

1 + 1 = 2

= 2 cos (x) sin^{2} (x)

= cos (x) cos (2 x) - 2 sin^{2} (x) cos (x)

= cos (x) cos (2 x) - 2 sin^{2} (x) cos (x)

= cos (x) cos (2 x) - 2 sin^{2} (x) cos (x)

Use the Double Angle identity:

cos (2 x) = 2 cos^{2} (x) - 1

= (2 cos^{2} (x) - 1) cos (x) - 2 sin^{2} (x) cos (x)

Use the Pythagorean identity:

cos^{2} (x) + sin^{2} (x) = 1

sin^{2} (x) = 1 - cos^{2} (x)

= (2 cos^{2} (x) - 1) cos (x) - 2 (1 - cos^{2} (x)) cos (x)

Expand

(2 cos^{2} (x) - 1) cos (x) - 2 (1 - cos^{2} (x)) cos (x) : 4 cos^{3} (x) - 3 cos (x)

(2 cos^{2} (x) - 1) cos (x) - 2 (1 - cos^{2} (x)) cos (x)

= cos (x) (2 cos^{2} (x) - 1) - 2 cos (x) (1 - cos^{2} (x))

Expand

cos (x) (2 cos^{2} (x) - 1) : 2 cos^{3} (x) - cos (x)

cos (x) (2 cos^{2} (x) - 1)

Apply the distributive law:

a (b - c) = ab - a c

a = cos (x), b = 2 cos^{2} (x), c = 1

= cos (x) 2 cos^{2} (x) - cos (x) 1

= 2 cos^{2} (x) cos (x) - 1 cos (x)

Simplify

2 cos^{2} (x) cos (x) - 1 \cdot cos (x) : 2 cos^{3} (x) - cos (x)

2 cos^{2} (x) cos (x) - 1 cos (x)

2 cos^{2} (x) cos (x) = 2 cos^{3} (x)

2 cos^{2} (x) cos (x)

Apply exponent rule:

a^{b} \cdot a^{c} = a^{b + c}

cos^{2} (x) cos (x) = cos^{2 + 1} (x)

= 2 cos^{2 + 1} (x)

Add the numbers:

2 + 1 = 3

= 2 cos^{3} (x)

1 \cdot cos (x) = cos (x)

1 cos (x)

Multiply:

1 \cdot cos (x) = cos (x)

= cos (x)

= 2 cos^{3} (x) - cos (x)

= 2 cos^{3} (x) - cos (x)

= 2 cos^{3} (x) - cos (x) - 2 (1 - cos^{2} (x)) cos (x)

Expand

- 2 cos (x) (1 - cos^{2} (x)) : - 2 cos (x) + 2 cos^{3} (x)

- 2 cos (x) (1 - cos^{2} (x))

Apply the distributive law:

a (b - c) = ab - a c

a = - 2 cos (x), b = 1, c = cos^{2} (x)

= - 2 cos (x) 1 - (- 2 cos (x)) cos^{2} (x)

Apply minus-plus rules

- (- a) = a

= - 2 \cdot 1 cos (x) + 2 cos^{2} (x) cos (x)

Simplify

- 2 \cdot 1 \cdot cos (x) + 2 cos^{2} (x) cos (x) : - 2 cos (x) + 2 cos^{3} (x)

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

2 \cdot 1 \cdot cos (x) = 2 cos (x)

2 \cdot 1 cos (x)

Multiply the numbers:

2 \cdot 1 = 2

= 2 cos (x)

2 cos^{2} (x) cos (x) = 2 cos^{3} (x)

2 cos^{2} (x) cos (x)

Apply exponent rule:

a^{b} \cdot a^{c} = a^{b + c}

cos^{2} (x) cos (x) = cos^{2 + 1} (x)

= 2 cos^{2 + 1} (x)

Add the numbers:

2 + 1 = 3

= 2 cos^{3} (x)

= - 2 cos (x) + 2 cos^{3} (x)

= - 2 cos (x) + 2 cos^{3} (x)

= 2 cos^{3} (x) - cos (x) - 2 cos (x) + 2 cos^{3} (x)

Simplify

2 cos^{3} (x) - cos (x) - 2 cos (x) + 2 cos^{3} (x) : 4 cos^{3} (x) - 3 cos (x)

2 cos^{3} (x) - cos (x) - 2 cos (x) + 2 cos^{3} (x)

Group like terms

= 2 cos^{3} (x) + 2 cos^{3} (x) - cos (x) - 2 cos (x)

Add similar elements:

2 cos^{3} (x) + 2 cos^{3} (x) = 4 cos^{3} (x)

= 4 cos^{3} (x) - cos (x) - 2 cos (x)

Add similar elements:

- cos (x) - 2 cos (x) = - 3 cos (x)

= 4 cos^{3} (x) - 3 cos (x)

= 4 cos^{3} (x) - 3 cos (x)

= 4 cos^{3} (x) - 3 cos (x)

= \frac{3}{4} cos (x) + \frac{1}{4} (4 cos^{3} (x) - 3 cos (x))

Simplify

\frac{3}{4} cos (x) + \frac{1}{4} (4 cos^{3} (x) - 3 cos (x)) : cos^{3} (x)

\frac{3}{4} cos (x) + \frac{1}{4} (4 cos^{3} (x) - 3 cos (x))

Expand

\frac{1}{4} (4 cos^{3} (x) - 3 cos (x)) : cos^{3} (x) - \frac{3}{4} cos (x)

\frac{1}{4} (4 cos^{3} (x) - 3 cos (x))

Apply the distributive law:

a (b - c) = ab - a c

a = \frac{1}{4}, b = 4 cos^{3} (x), c = 3 cos (x)

= \frac{1}{4} \cdot 4 cos^{3} (x) - \frac{1}{4} \cdot 3 cos (x)

= 4 \cdot \frac{1}{4} cos^{3} (x) - 3 \cdot \frac{1}{4} cos (x)

Simplify

4 \cdot \frac{1}{4} cos^{3} (x) - 3 \cdot \frac{1}{4} cos (x) : cos^{3} (x) - \frac{3}{4} cos (x)

4 \cdot \frac{1}{4} cos^{3} (x) - 3 \cdot \frac{1}{4} cos (x)

4 \cdot \frac{1}{4} cos^{3} (x) = cos^{3} (x)

4 \cdot \frac{1}{4} cos^{3} (x)

Multiply fractions:

a \cdot \frac{b}{c} = \frac{a \cdot b}{c}

= \frac{1 \cdot 4}{4} cos^{3} (x)

Cancel the common factor:

4

= cos^{3} (x) \cdot 1

Multiply:

cos^{3} (x) \cdot 1 = cos^{3} (x)

= cos^{3} (x)

3 \cdot \frac{1}{4} cos (x) = \frac{3}{4} cos (x)

3 \cdot \frac{1}{4} cos (x)

Multiply fractions:

a \cdot \frac{b}{c} = \frac{a \cdot b}{c}

= \frac{1 \cdot 3}{4} cos (x)

Multiply the numbers:

1 \cdot 3 = 3

= \frac{3}{4} cos (x)

= cos^{3} (x) - \frac{3}{4} cos (x)

= cos^{3} (x) - \frac{3}{4} cos (x)

= \frac{3}{4} cos (x) + cos^{3} (x) - \frac{3}{4} cos (x)

Simplify

\frac{3}{4} cos (x) + cos^{3} (x) - \frac{3}{4} cos (x) : cos^{3} (x)

\frac{3}{4} cos (x) + cos^{3} (x) - \frac{3}{4} cos (x)

Group like terms

= \frac{3}{4} cos (x) - \frac{3}{4} cos (x) + cos^{3} (x)

Add similar elements:

\frac{3}{4} cos (x) - \frac{3}{4} cos (x) = 0

\frac{3}{4} cos (x) - \frac{3}{4} cos (x)

Factor out common term

cos (x)

= cos (x) (\frac{3}{4} - \frac{3}{4})

\frac{3}{4} - \frac{3}{4} = 0

\frac{3}{4} - \frac{3}{4}

Since the denominators are equal, combine the fractions:

\frac{a}{c} \pm \frac{b}{c} = \frac{a \pm b}{c}

= \frac{3 - 3}{4}

Refine

= 0

= 0

= cos^{3} (x)

= cos^{3} (x)

= cos^{3} (x)

= cos^{3} (x)

We showed that the two sides could take the same form

\Rightarrow True

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Frequently Asked Questions (FAQ)

Is cos^3(x)= 3/4 cos(x)+1/4 cos(3x) ?
The answer to whether cos^3(x)= 3/4 cos(x)+1/4 cos(3x) is True