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Popular Trigonometry >

sin^4(x)+cos^4(x)= 7/9

Solution

sin^{4} (x) + cos^{4} (x) = \frac{7}{9}

Solution

x = 0.36486 \dots + 2 πn, x = 2 π - 0.36486 \dots + 2 πn, x = 2.77672 \dots + 2 πn, x = - 2.77672 \dots + 2 πn, x = 1.20593 \dots + 2 πn, x = 2 π - 1.20593 \dots + 2 πn, x = 1.93566 \dots + 2 πn, x = - 1.93566 \dots + 2 πn

Degrees

x = 20.90515 \dots^{\circ} + 36 0^{\circ} n, x = 339.09484 \dots^{\circ} + 36 0^{\circ} n, x = 159.09484 \dots^{\circ} + 36 0^{\circ} n, x = - 159.09484 \dots^{\circ} + 36 0^{\circ} n, x = 69.09484 \dots^{\circ} + 36 0^{\circ} n, x = 290.90515 \dots^{\circ} + 36 0^{\circ} n, x = 110.90515 \dots^{\circ} + 36 0^{\circ} n, x = - 110.90515 \dots^{\circ} + 36 0^{\circ} n

Solution steps

sin^{4} (x) + cos^{4} (x) = \frac{7}{9}

Subtract

\frac{7}{9}

from both sides

sin^{4} (x) + cos^{4} (x) - \frac{7}{9} = 0

Simplify

sin^{4} (x) + cos^{4} (x) - \frac{7}{9} : \frac{9 sin ^{4} ( x ) + 9 cos ^{4} ( x ) - 7}{9}

sin^{4} (x) + cos^{4} (x) - \frac{7}{9}

Convert element to fraction:

sin^{4} (x) = \frac{sin ^{4} ( x ) 9}{9}, cos^{4} (x) = \frac{cos ^{4} ( x ) 9}{9}

= \frac{sin ^{4} ( x ) \cdot 9}{9} + \frac{cos ^{4} ( x ) \cdot 9}{9} - \frac{7}{9}

Since the denominators are equal, combine the fractions:

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

= \frac{sin ^{4} ( x ) \cdot 9 + cos ^{4} ( x ) \cdot 9 - 7}{9}

\frac{9 sin ^{4} ( x ) + 9 cos ^{4} ( x ) - 7}{9} = 0

\frac{f ( x )}{g ( x )} = 0 \Rightarrow f (x) = 0

9 sin^{4} (x) + 9 cos^{4} (x) - 7 = 0

Apply exponent rule:

a^{b} = a^{2} a^{b - 2}

- 7 + 9 cos^{4} (x) + 9 sin^{2} (x) sin^{2} (x) = 0

Rewrite using trig identities

- 7 + 9 cos^{4} (x) + 9 sin^{2} (x) sin^{2} (x)

Use the Pythagorean identity:

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

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

= - 7 + 9 cos^{4} (x) + 9 (1 - cos^{2} (x)) (1 - cos^{2} (x))

Simplify

- 7 + 9 cos^{4} (x) + 9 (1 - cos^{2} (x)) (1 - cos^{2} (x)) : 18 cos^{4} (x) - 18 cos^{2} (x) + 2

- 7 + 9 cos^{4} (x) + 9 (1 - cos^{2} (x)) (1 - cos^{2} (x))

9 (1 - cos^{2} (x)) (1 - cos^{2} (x)) = 9 (1 - cos^{2} (x))^{2}

9 (1 - cos^{2} (x)) (1 - cos^{2} (x))

Apply exponent rule:

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

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

= 9 (1 - cos^{2} (x))^{1 + 1}

Add the numbers:

1 + 1 = 2

= 9 (1 - cos^{2} (x))^{2}

= - 7 + 9 cos^{4} (x) + 9 (- cos^{2} (x) + 1)^{2}

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

Apply Perfect Square Formula:

(a - b)^{2} = a^{2} - 2 ab + b^{2}

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

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

Simplify

1^{2} - 2 \cdot 1 \cdot cos^{2} (x) + (cos^{2} (x))^{2} : 1 - 2 cos^{2} (x) + cos^{4} (x)

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

Apply rule

1^{a} = 1

1^{2} = 1

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

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

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

Multiply the numbers:

2 \cdot 1 = 2

= 2 cos^{2} (x)

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

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

Apply exponent rule:

(a^{b})^{c} = a^{b c}

= cos^{2 \cdot 2} (x)

Multiply the numbers:

2 \cdot 2 = 4

= cos^{4} (x)

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

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

= - 7 + 9 cos^{4} (x) + 9 (1 - 2 cos^{2} (x) + cos^{4} (x))

Expand

9 (1 - 2 cos^{2} (x) + cos^{4} (x)) : 9 - 18 cos^{2} (x) + 9 cos^{4} (x)

9 (1 - 2 cos^{2} (x) + cos^{4} (x))

Distribute parentheses

= 9 \cdot 1 + 9 (- 2 cos^{2} (x)) + 9 cos^{4} (x)

Apply minus-plus rules

+ (- a) = - a

= 9 \cdot 1 - 9 \cdot 2 cos^{2} (x) + 9 cos^{4} (x)

Simplify

9 \cdot 1 - 9 \cdot 2 cos^{2} (x) + 9 cos^{4} (x) : 9 - 18 cos^{2} (x) + 9 cos^{4} (x)

9 \cdot 1 - 9 \cdot 2 cos^{2} (x) + 9 cos^{4} (x)

Multiply the numbers:

9 \cdot 1 = 9

= 9 - 9 \cdot 2 cos^{2} (x) + 9 cos^{4} (x)

Multiply the numbers:

9 \cdot 2 = 18

= 9 - 18 cos^{2} (x) + 9 cos^{4} (x)

= 9 - 18 cos^{2} (x) + 9 cos^{4} (x)

= - 7 + 9 cos^{4} (x) + 9 - 18 cos^{2} (x) + 9 cos^{4} (x)

Simplify

- 7 + 9 cos^{4} (x) + 9 - 18 cos^{2} (x) + 9 cos^{4} (x) : 18 cos^{4} (x) - 18 cos^{2} (x) + 2

- 7 + 9 cos^{4} (x) + 9 - 18 cos^{2} (x) + 9 cos^{4} (x)

Group like terms

= 9 cos^{4} (x) - 18 cos^{2} (x) + 9 cos^{4} (x) - 7 + 9

Add similar elements:

9 cos^{4} (x) + 9 cos^{4} (x) = 18 cos^{4} (x)

= 18 cos^{4} (x) - 18 cos^{2} (x) - 7 + 9

Add/Subtract the numbers:

- 7 + 9 = 2

= 18 cos^{4} (x) - 18 cos^{2} (x) + 2

= 18 cos^{4} (x) - 18 cos^{2} (x) + 2

= 18 cos^{4} (x) - 18 cos^{2} (x) + 2

2 - 18 cos^{2} (x) + 18 cos^{4} (x) = 0

Solve by substitution

2 - 18 cos^{2} (x) + 18 cos^{4} (x) = 0

Let:

cos (x) = u

2 - 18 u^{2} + 18 u^{4} = 0

2 - 18 u^{2} + 18 u^{4} = 0 : u = \frac{3 + 5}{6}, u = - \frac{3 + 5}{6}, u = \frac{3 - 5}{6}, u = - \frac{3 - 5}{6}

2 - 18 u^{2} + 18 u^{4} = 0

Write in the standard form

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

18 u^{4} - 18 u^{2} + 2 = 0

Rewrite the equation with

v = u^{2}

and

v^{2} = u^{4}

18 v^{2} - 18 v + 2 = 0

Solve

18 v^{2} - 18 v + 2 = 0 : v = \frac{3 + 5}{6}, v = \frac{3 - 5}{6}

18 v^{2} - 18 v + 2 = 0

Solve with the quadratic formula

18 v^{2} - 18 v + 2 = 0

Quadratic Equation Formula:

For

a = 18, b = - 18, c = 2

v_{1, 2} = \frac{- ( - 18 ) \pm ( - 18 ) ^{2} - 4 \cdot 18 \cdot 2}{2 \cdot 18}

v_{1, 2} = \frac{- ( - 18 ) \pm ( - 18 ) ^{2} - 4 \cdot 18 \cdot 2}{2 \cdot 18}

(- 18)^{2} - 4 \cdot 18 \cdot 2 = 65

(- 18)^{2} - 4 \cdot 18 \cdot 2

Apply exponent rule:

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

n

is even

(- 18)^{2} = 1 8^{2}

= 1 8^{2} - 4 \cdot 18 \cdot 2

Multiply the numbers:

4 \cdot 18 \cdot 2 = 144

= 1 8^{2} - 144

1 8^{2} = 324

= 324 - 144

Subtract the numbers:

324 - 144 = 180

= 180

Prime factorization of

180 : 2^{2} \cdot 3^{2} \cdot 5

180

180

divides by

2180 = 90 \cdot 2

= 2 \cdot 90

90

divides by

290 = 45 \cdot 2

= 2 \cdot 2 \cdot 45

45

divides by

345 = 15 \cdot 3

= 2 \cdot 2 \cdot 3 \cdot 15

15

divides by

315 = 5 \cdot 3

= 2 \cdot 2 \cdot 3 \cdot 3 \cdot 5

2, 3, 5

are all prime numbers, therefore no further factorization is possible

= 2 \cdot 2 \cdot 3 \cdot 3 \cdot 5

= 2^{2} \cdot 3^{2} \cdot 5

= 2^{2} \cdot 3^{2} \cdot 5

Apply radical rule:

n ab = n a n b

= 5 2^{2} 3^{2}

Apply radical rule:

n a^{n} = a

2^{2} = 2

= 25 3^{2}

Apply radical rule:

n a^{n} = a

3^{2} = 3

= 2 \cdot 35

Refine

= 65

v_{1, 2} = \frac{- ( - 18 ) \pm 6 5}{2 \cdot 18}

Separate the solutions

v_{1} = \frac{- ( - 18 ) + 6 5}{2 \cdot 18}, v_{2} = \frac{- ( - 18 ) - 6 5}{2 \cdot 18}

v = \frac{- ( - 18 ) + 6 5}{2 \cdot 18} : \frac{3 + 5}{6}

\frac{- ( - 18 ) + 6 5}{2 \cdot 18}

Apply rule

- (- a) = a

= \frac{18 + 6 5}{2 \cdot 18}

Multiply the numbers:

2 \cdot 18 = 36

= \frac{18 + 6 5}{36}

Factor

18 + 65 : 6 (3 + 5)

18 + 65

Rewrite as

= 6 \cdot 3 + 65

Factor out common term

6

= 6 (3 + 5)

= \frac{6 ( 3 + 5 )}{36}

Cancel the common factor:

6

= \frac{3 + 5}{6}

v = \frac{- ( - 18 ) - 6 5}{2 \cdot 18} : \frac{3 - 5}{6}

\frac{- ( - 18 ) - 6 5}{2 \cdot 18}

Apply rule

- (- a) = a

= \frac{18 - 6 5}{2 \cdot 18}

Multiply the numbers:

2 \cdot 18 = 36

= \frac{18 - 6 5}{36}

Factor

18 - 65 : 6 (3 - 5)

18 - 65

Rewrite as

= 6 \cdot 3 - 65

Factor out common term

6

= 6 (3 - 5)

= \frac{6 ( 3 - 5 )}{36}

Cancel the common factor:

6

= \frac{3 - 5}{6}

The solutions to the quadratic equation are:

v = \frac{3 + 5}{6}, v = \frac{3 - 5}{6}

v = \frac{3 + 5}{6}, v = \frac{3 - 5}{6}

Substitute back

v = u^{2},

solve for

u

Solve

u^{2} = \frac{3 + 5}{6} : u = \frac{3 + 5}{6}, u = - \frac{3 + 5}{6}

u^{2} = \frac{3 + 5}{6}

For

x^{2} = f (a)

the solutions are

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

u = \frac{3 + 5}{6}, u = - \frac{3 + 5}{6}

Solve

u^{2} = \frac{3 - 5}{6} : u = \frac{3 - 5}{6}, u = - \frac{3 - 5}{6}

u^{2} = \frac{3 - 5}{6}

For

x^{2} = f (a)

the solutions are

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

u = \frac{3 - 5}{6}, u = - \frac{3 - 5}{6}

The solutions are

u = \frac{3 + 5}{6}, u = - \frac{3 + 5}{6}, u = \frac{3 - 5}{6}, u = - \frac{3 - 5}{6}

Substitute back

u = cos (x)

cos (x) = \frac{3 + 5}{6}, cos (x) = - \frac{3 + 5}{6}, cos (x) = \frac{3 - 5}{6}, cos (x) = - \frac{3 - 5}{6}

cos (x) = \frac{3 + 5}{6}, cos (x) = - \frac{3 + 5}{6}, cos (x) = \frac{3 - 5}{6}, cos (x) = - \frac{3 - 5}{6}

cos (x) = \frac{3 + 5}{6} : x = arccos \frac{3 + 5}{6} + 2 πn, x = 2 π - arccos \frac{3 + 5}{6} + 2 πn

cos (x) = \frac{3 + 5}{6}

Apply trig inverse properties

cos (x) = \frac{3 + 5}{6}

General solutions for

cos (x) = \frac{3 + 5}{6}

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

x = arccos \frac{3 + 5}{6} + 2 πn, x = 2 π - arccos \frac{3 + 5}{6} + 2 πn

x = arccos \frac{3 + 5}{6} + 2 πn, x = 2 π - arccos \frac{3 + 5}{6} + 2 πn

cos (x) = - \frac{3 + 5}{6} : x = arccos - \frac{3 + 5}{6} + 2 πn, x = - arccos - \frac{3 + 5}{6} + 2 πn

cos (x) = - \frac{3 + 5}{6}

Apply trig inverse properties

cos (x) = - \frac{3 + 5}{6}

General solutions for

cos (x) = - \frac{3 + 5}{6}

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

x = arccos - \frac{3 + 5}{6} + 2 πn, x = - arccos - \frac{3 + 5}{6} + 2 πn

x = arccos - \frac{3 + 5}{6} + 2 πn, x = - arccos - \frac{3 + 5}{6} + 2 πn

cos (x) = \frac{3 - 5}{6} : x = arccos \frac{3 - 5}{6} + 2 πn, x = 2 π - arccos \frac{3 - 5}{6} + 2 πn

cos (x) = \frac{3 - 5}{6}

Apply trig inverse properties

cos (x) = \frac{3 - 5}{6}

General solutions for

cos (x) = \frac{3 - 5}{6}

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

x = arccos \frac{3 - 5}{6} + 2 πn, x = 2 π - arccos \frac{3 - 5}{6} + 2 πn

x = arccos \frac{3 - 5}{6} + 2 πn, x = 2 π - arccos \frac{3 - 5}{6} + 2 πn

cos (x) = - \frac{3 - 5}{6} : x = arccos - \frac{3 - 5}{6} + 2 πn, x = - arccos - \frac{3 - 5}{6} + 2 πn

cos (x) = - \frac{3 - 5}{6}

Apply trig inverse properties

cos (x) = - \frac{3 - 5}{6}

General solutions for

cos (x) = - \frac{3 - 5}{6}

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

x = arccos - \frac{3 - 5}{6} + 2 πn, x = - arccos - \frac{3 - 5}{6} + 2 πn

x = arccos - \frac{3 - 5}{6} + 2 πn, x = - arccos - \frac{3 - 5}{6} + 2 πn

Combine all the solutions

x = arccos \frac{3 + 5}{6} + 2 πn, x = 2 π - arccos \frac{3 + 5}{6} + 2 πn, x = arccos - \frac{3 + 5}{6} + 2 πn, x = - arccos - \frac{3 + 5}{6} + 2 πn, x = arccos \frac{3 - 5}{6} + 2 πn, x = 2 π - arccos \frac{3 - 5}{6} + 2 πn, x = arccos - \frac{3 - 5}{6} + 2 πn, x = - arccos - \frac{3 - 5}{6} + 2 πn

Show solutions in decimal form

x = 0.36486 \dots + 2 πn, x = 2 π - 0.36486 \dots + 2 πn, x = 2.77672 \dots + 2 πn, x = - 2.77672 \dots + 2 πn, x = 1.20593 \dots + 2 πn, x = 2 π - 1.20593 \dots + 2 πn, x = 1.93566 \dots + 2 πn, x = - 1.93566 \dots + 2 πn

Graph

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