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Popular >

cos^2(x)+cos(2x)=0

Solution

cos^{2} (x) + cos (2 x) = 0

Solution

x = 0.95531 \dots + 2 πn, x = 2 π - 0.95531 \dots + 2 πn, x = 2.18627 \dots + 2 πn, x = - 2.18627 \dots + 2 πn

+1

Degrees

x = 54.73561 \dots^{\circ} + 36 0^{\circ} n, x = 305.26438 \dots^{\circ} + 36 0^{\circ} n, x = 125.26438 \dots^{\circ} + 36 0^{\circ} n, x = - 125.26438 \dots^{\circ} + 36 0^{\circ} n

Solution steps

cos^{2} (x) + cos (2 x) = 0

Rewrite using trig identities

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

Use the Double Angle identity:

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

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

Simplify

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

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

Group like terms

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

Add similar elements:

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

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

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

- 1 + 3 cos^{2} (x) = 0

Solve by substitution

- 1 + 3 cos^{2} (x) = 0

Let:

cos (x) = u

- 1 + 3 u^{2} = 0

- 1 + 3 u^{2} = 0 : u = \frac{1}{3}, u = - \frac{1}{3}

- 1 + 3 u^{2} = 0

Move

1

to the right side

- 1 + 3 u^{2} = 0

Add

1

to both sides

- 1 + 3 u^{2} + 1 = 0 + 1

Simplify

3 u^{2} = 1

3 u^{2} = 1

Divide both sides by

3

3 u^{2} = 1

Divide both sides by

3

\frac{3 u ^{2}}{3} = \frac{1}{3}

Simplify

u^{2} = \frac{1}{3}

u^{2} = \frac{1}{3}

For

x^{2} = f (a)

the solutions are

x = f (a), - f (a)

u = \frac{1}{3}, u = - \frac{1}{3}

Substitute back

u = cos (x)

cos (x) = \frac{1}{3}, cos (x) = - \frac{1}{3}

cos (x) = \frac{1}{3}, cos (x) = - \frac{1}{3}

cos (x) = \frac{1}{3} : x = arccos (\frac{1}{3}) + 2 πn, x = 2 π - arccos (\frac{1}{3}) + 2 πn

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

Apply trig inverse properties

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

General solutions for

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

cos (x) = a \Rightarrow x = arccos (a) + 2 πn, x = 2 π - arccos (a) + 2 πn

x = arccos (\frac{1}{3}) + 2 πn, x = 2 π - arccos (\frac{1}{3}) + 2 πn

x = arccos (\frac{1}{3}) + 2 πn, x = 2 π - arccos (\frac{1}{3}) + 2 πn

cos (x) = - \frac{1}{3} : x = arccos (- \frac{1}{3}) + 2 πn, x = - arccos (- \frac{1}{3}) + 2 πn

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

Apply trig inverse properties

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

General solutions for

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

cos (x) = - a \Rightarrow x = arccos (- a) + 2 πn, x = - arccos (- a) + 2 πn

x = arccos (- \frac{1}{3}) + 2 πn, x = - arccos (- \frac{1}{3}) + 2 πn

x = arccos (- \frac{1}{3}) + 2 πn, x = - arccos (- \frac{1}{3}) + 2 πn

Combine all the solutions

x = arccos (\frac{1}{3}) + 2 πn, x = 2 π - arccos (\frac{1}{3}) + 2 πn, x = arccos (- \frac{1}{3}) + 2 πn, x = - arccos (- \frac{1}{3}) + 2 πn

Show solutions in decimal form

x = 0.95531 \dots + 2 πn, x = 2 π - 0.95531 \dots + 2 πn, x = 2.18627 \dots + 2 πn, x = - 2.18627 \dots + 2 πn

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