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Popular >

2cos^4(x)+8sin^2(x)=5

Solution

2 cos^{4} (x) + 8 sin^{2} (x) = 5

Solution

x = 0.86689 \dots + 2 πn, x = 2 π - 0.86689 \dots + 2 πn, x = 2.27469 \dots + 2 πn, x = - 2.27469 \dots + 2 πn

Degrees

x = 49.66956 \dots^{\circ} + 36 0^{\circ} n, x = 310.33043 \dots^{\circ} + 36 0^{\circ} n, x = 130.33043 \dots^{\circ} + 36 0^{\circ} n, x = - 130.33043 \dots^{\circ} + 36 0^{\circ} n

Solution steps

2 cos^{4} (x) + 8 sin^{2} (x) = 5

Subtract

5

from both sides

2 cos^{4} (x) + 8 sin^{2} (x) - 5 = 0

Rewrite using trig identities

- 5 + 2 cos^{4} (x) + 8 sin^{2} (x)

Use the Pythagorean identity:

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

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

= - 5 + 2 cos^{4} (x) + 8 (1 - cos^{2} (x))

Simplify

- 5 + 2 cos^{4} (x) + 8 (1 - cos^{2} (x)) : 2 cos^{4} (x) - 8 cos^{2} (x) + 3

- 5 + 2 cos^{4} (x) + 8 (1 - cos^{2} (x))

Expand

8 (1 - cos^{2} (x)) : 8 - 8 cos^{2} (x)

8 (1 - cos^{2} (x))

Apply the distributive law:

a (b - c) = ab - a c

a = 8, b = 1, c = cos^{2} (x)

= 8 \cdot 1 - 8 cos^{2} (x)

Multiply the numbers:

8 \cdot 1 = 8

= 8 - 8 cos^{2} (x)

= - 5 + 2 cos^{4} (x) + 8 - 8 cos^{2} (x)

Simplify

- 5 + 2 cos^{4} (x) + 8 - 8 cos^{2} (x) : 2 cos^{4} (x) - 8 cos^{2} (x) + 3

- 5 + 2 cos^{4} (x) + 8 - 8 cos^{2} (x)

Group like terms

= 2 cos^{4} (x) - 8 cos^{2} (x) - 5 + 8

Add/Subtract the numbers:

- 5 + 8 = 3

= 2 cos^{4} (x) - 8 cos^{2} (x) + 3

= 2 cos^{4} (x) - 8 cos^{2} (x) + 3

= 2 cos^{4} (x) - 8 cos^{2} (x) + 3

3 + 2 cos^{4} (x) - 8 cos^{2} (x) = 0

Solve by substitution

3 + 2 cos^{4} (x) - 8 cos^{2} (x) = 0

Let:

cos (x) = u

3 + 2 u^{4} - 8 u^{2} = 0

3 + 2 u^{4} - 8 u^{2} = 0 : u = \frac{4 + 10}{2}, u = - \frac{4 + 10}{2}, u = \frac{4 - 10}{2}, u = - \frac{4 - 10}{2}

3 + 2 u^{4} - 8 u^{2} = 0

Write in the standard form

a_{n} x^{n} + \dots + a_{1} x + a_{0} = 0

2 u^{4} - 8 u^{2} + 3 = 0

Rewrite the equation with

v = u^{2}

and

v^{2} = u^{4}

2 v^{2} - 8 v + 3 = 0

Solve

2 v^{2} - 8 v + 3 = 0 : v = \frac{4 + 10}{2}, v = \frac{4 - 10}{2}

2 v^{2} - 8 v + 3 = 0

Solve with the quadratic formula

2 v^{2} - 8 v + 3 = 0

Quadratic Equation Formula:

For

a = 2, b = - 8, c = 3

v_{1, 2} = \frac{- ( - 8 ) \pm ( - 8 ) ^{2} - 4 \cdot 2 \cdot 3}{2 \cdot 2}

v_{1, 2} = \frac{- ( - 8 ) \pm ( - 8 ) ^{2} - 4 \cdot 2 \cdot 3}{2 \cdot 2}

(- 8)^{2} - 4 \cdot 2 \cdot 3 = 210

(- 8)^{2} - 4 \cdot 2 \cdot 3

Apply exponent rule:

(- a)^{n} = a^{n},

n

is even

(- 8)^{2} = 8^{2}

= 8^{2} - 4 \cdot 2 \cdot 3

Multiply the numbers:

4 \cdot 2 \cdot 3 = 24

= 8^{2} - 24

8^{2} = 64

= 64 - 24

Subtract the numbers:

64 - 24 = 40

= 40

Prime factorization of

40 : 2^{3} \cdot 5

40

40

divides by

240 = 20 \cdot 2

= 2 \cdot 20

20

divides by

220 = 10 \cdot 2

= 2 \cdot 2 \cdot 10

10

divides by

210 = 5 \cdot 2

= 2 \cdot 2 \cdot 2 \cdot 5

2, 5

are all prime numbers, therefore no further factorization is possible

= 2 \cdot 2 \cdot 2 \cdot 5

= 2^{3} \cdot 5

= 2^{3} \cdot 5

Apply exponent rule:

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

= 2^{2} \cdot 2 \cdot 5

Apply radical rule:

= 2^{2} 2 \cdot 5

Apply radical rule:

2^{2} = 2

= 2 2 \cdot 5

Refine

= 210

v_{1, 2} = \frac{- ( - 8 ) \pm 2 10}{2 \cdot 2}

Separate the solutions

v_{1} = \frac{- ( - 8 ) + 2 10}{2 \cdot 2}, v_{2} = \frac{- ( - 8 ) - 2 10}{2 \cdot 2}

v = \frac{- ( - 8 ) + 2 10}{2 \cdot 2} : \frac{4 + 10}{2}

\frac{- ( - 8 ) + 2 10}{2 \cdot 2}

Apply rule

- (- a) = a

= \frac{8 + 2 10}{2 \cdot 2}

Multiply the numbers:

2 \cdot 2 = 4

= \frac{8 + 2 10}{4}

Factor

8 + 210 : 2 (4 + 10)

8 + 210

Rewrite as

= 2 \cdot 4 + 210

Factor out common term

2

= 2 (4 + 10)

= \frac{2 ( 4 + 10 )}{4}

Cancel the common factor:

2

= \frac{4 + 10}{2}

v = \frac{- ( - 8 ) - 2 10}{2 \cdot 2} : \frac{4 - 10}{2}

\frac{- ( - 8 ) - 2 10}{2 \cdot 2}

Apply rule

- (- a) = a

= \frac{8 - 2 10}{2 \cdot 2}

Multiply the numbers:

2 \cdot 2 = 4

= \frac{8 - 2 10}{4}

Factor

8 - 210 : 2 (4 - 10)

8 - 210

Rewrite as

= 2 \cdot 4 - 210

Factor out common term

2

= 2 (4 - 10)

= \frac{2 ( 4 - 10 )}{4}

Cancel the common factor:

2

= \frac{4 - 10}{2}

The solutions to the quadratic equation are:

v = \frac{4 + 10}{2}, v = \frac{4 - 10}{2}

v = \frac{4 + 10}{2}, v = \frac{4 - 10}{2}

Substitute back

v = u^{2},

solve for

u

Solve

u^{2} = \frac{4 + 10}{2} : u = \frac{4 + 10}{2}, u = - \frac{4 + 10}{2}

u^{2} = \frac{4 + 10}{2}

For

x^{2} = f (a)

the solutions are

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

u = \frac{4 + 10}{2}, u = - \frac{4 + 10}{2}

Solve

u^{2} = \frac{4 - 10}{2} : u = \frac{4 - 10}{2}, u = - \frac{4 - 10}{2}

u^{2} = \frac{4 - 10}{2}

For

x^{2} = f (a)

the solutions are

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

u = \frac{4 - 10}{2}, u = - \frac{4 - 10}{2}

The solutions are

u = \frac{4 + 10}{2}, u = - \frac{4 + 10}{2}, u = \frac{4 - 10}{2}, u = - \frac{4 - 10}{2}

Substitute back

u = cos (x)

cos (x) = \frac{4 + 10}{2}, cos (x) = - \frac{4 + 10}{2}, cos (x) = \frac{4 - 10}{2}, cos (x) = - \frac{4 - 10}{2}

cos (x) = \frac{4 + 10}{2}, cos (x) = - \frac{4 + 10}{2}, cos (x) = \frac{4 - 10}{2}, cos (x) = - \frac{4 - 10}{2}

cos (x) = \frac{4 + 10}{2} :

No Solution

cos (x) = \frac{4 + 10}{2}

- 1 \leq cos (x) \leq 1

No Solution

cos (x) = - \frac{4 + 10}{2} :

No Solution

cos (x) = - \frac{4 + 10}{2}

- 1 \leq cos (x) \leq 1

No Solution

cos (x) = \frac{4 - 10}{2} : x = arccos \frac{4 - 10}{2} + 2 πn, x = 2 π - arccos \frac{4 - 10}{2} + 2 πn

cos (x) = \frac{4 - 10}{2}

Apply trig inverse properties

cos (x) = \frac{4 - 10}{2}

General solutions for

cos (x) = \frac{4 - 10}{2}

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

x = arccos \frac{4 - 10}{2} + 2 πn, x = 2 π - arccos \frac{4 - 10}{2} + 2 πn

x = arccos \frac{4 - 10}{2} + 2 πn, x = 2 π - arccos \frac{4 - 10}{2} + 2 πn

cos (x) = - \frac{4 - 10}{2} : x = arccos - \frac{4 - 10}{2} + 2 πn, x = - arccos - \frac{4 - 10}{2} + 2 πn

cos (x) = - \frac{4 - 10}{2}

Apply trig inverse properties

cos (x) = - \frac{4 - 10}{2}

General solutions for

cos (x) = - \frac{4 - 10}{2}

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

x = arccos - \frac{4 - 10}{2} + 2 πn, x = - arccos - \frac{4 - 10}{2} + 2 πn

x = arccos - \frac{4 - 10}{2} + 2 πn, x = - arccos - \frac{4 - 10}{2} + 2 πn

Combine all the solutions

x = arccos \frac{4 - 10}{2} + 2 πn, x = 2 π - arccos \frac{4 - 10}{2} + 2 πn, x = arccos - \frac{4 - 10}{2} + 2 πn, x = - arccos - \frac{4 - 10}{2} + 2 πn

Show solutions in decimal form

x = 0.86689 \dots + 2 πn, x = 2 π - 0.86689 \dots + 2 πn, x = 2.27469 \dots + 2 πn, x = - 2.27469 \dots + 2 πn

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