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Popular Trigonometry >

4cos^2(x)+2cos(x)-2sqrt(2)cos(x)=sqrt(2)

Solution

4 cos^{2} (x) + 2 cos (x) - 22 cos (x) = 2

Solution

x = \frac{π}{4} + 2 πn, x = \frac{7 π}{4} + 2 πn, x = \frac{2 π}{3} + 2 πn, x = \frac{4 π}{3} + 2 πn

Degrees

x = 4 5^{\circ} + 36 0^{\circ} n, x = 31 5^{\circ} + 36 0^{\circ} n, x = 12 0^{\circ} + 36 0^{\circ} n, x = 24 0^{\circ} + 36 0^{\circ} n

Solution steps

4 cos^{2} (x) + 2 cos (x) - 22 cos (x) = 2

Solve by substitution

4 cos^{2} (x) + 2 cos (x) - 22 cos (x) = 2

Let:

cos (x) = u

4 u^{2} + 2 u - 22 u = 2

4 u^{2} + 2 u - 22 u = 2 : u = \frac{2}{2}, u = - \frac{1}{2}

4 u^{2} + 2 u - 22 u = 2

Move

2

to the left side

4 u^{2} + 2 u - 22 u = 2

Subtract

2

from both sides

4 u^{2} + 2 u - 22 u - 2 = 2 - 2

Simplify

4 u^{2} + 2 u - 22 u - 2 = 0

4 u^{2} + 2 u - 22 u - 2 = 0

Write in the standard form

a x^{2} + b x + c = 0

4 u^{2} + (2 - 22) u - 2 = 0

Solve with the quadratic formula

4 u^{2} + (2 - 22) u - 2 = 0

Quadratic Equation Formula:

For

a = 4, b = 2 - 22, c = - 2

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

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

(2 - 22)^{2} - 4 \cdot 4 (- 2) = 2 + 22

(2 - 22)^{2} - 4 \cdot 4 (- 2)

Apply rule

- (- a) = a

= (2 - 22)^{2} + 4 \cdot 42

Multiply the numbers:

4 \cdot 4 = 16

= (2 - 22)^{2} + 162

Expand

(2 - 22)^{2} + 162 : 12 + 82

(2 - 22)^{2} + 162

(2 - 22)^{2} : 12 - 82

Apply Perfect Square Formula:

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

a = 2, b = 22

= 2^{2} - 2 \cdot 2 \cdot 22 + (22)^{2}

Simplify

2^{2} - 2 \cdot 2 \cdot 22 + (22)^{2} : 12 - 82

2^{2} - 2 \cdot 2 \cdot 22 + (22)^{2}

2^{2} = 4

2^{2}

2^{2} = 4

= 4

2 \cdot 2 \cdot 22 = 82

2 \cdot 2 \cdot 22

Multiply the numbers:

2 \cdot 2 \cdot 2 = 8

= 82

(22)^{2} = 8

(22)^{2}

Apply exponent rule:

(a \cdot b)^{n} = a^{n} b^{n}

= 2^{2} (2)^{2}

(2)^{2} : 2

Apply radical rule:

a = a^{\frac{1}{2}}

= (2^{\frac{1}{2}})^{2}

Apply exponent rule:

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

= 2^{\frac{1}{2} \cdot 2}

\frac{1}{2} \cdot 2 = 1

\frac{1}{2} \cdot 2

Multiply fractions:

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

= \frac{1 \cdot 2}{2}

Cancel the common factor:

2

= 1

= 2

= 2^{2} \cdot 2

Apply exponent rule:

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

2^{2} \cdot 2 = 2^{2 + 1}

= 2^{2 + 1}

Add the numbers:

2 + 1 = 3

= 2^{3}

2^{3} = 8

= 8

= 4 - 82 + 8

Add the numbers:

4 + 8 = 12

= 12 - 82

= 12 - 82

= 12 - 82 + 162

Add similar elements:

- 82 + 162 = 82

= 12 + 82

= 12 + 82

= 4 + 82 + 8

= 2 \cdot 2 + 82 + 8

= (2)^{2} (2)^{2} + 82 + (8)^{2}

8 = 22

8

Prime factorization of

8 : 2^{3}

8

8

divides by

28 = 4 \cdot 2

= 2 \cdot 4

4

divides by

24 = 2 \cdot 2

= 2 \cdot 2 \cdot 2

2

is a prime number, therefore no further factorization is possible

= 2 \cdot 2 \cdot 2

= 2^{3}

= 2^{3}

Apply exponent rule:

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

= 2^{2} \cdot 2

Apply radical rule:

= 2 2^{2}

Apply radical rule:

2^{2} = 2

= 22

= (2)^{2} (2)^{2} + 82 + (22)^{2}

222 \cdot 22 = 82

222 \cdot 22

Multiply the numbers:

2 \cdot 2 = 4

= 4222

Apply exponent rule:

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

222 = 2^{\frac{1}{2}} \cdot 2^{\frac{1}{2}} \cdot 2^{\frac{1}{2}} = 2^{\frac{1}{2} + \frac{1}{2} + \frac{1}{2}}

= 4 \cdot 2^{\frac{1}{2} + \frac{1}{2} + \frac{1}{2}}

2^{\frac{1}{2} + \frac{1}{2} + \frac{1}{2}} = 22

2^{\frac{1}{2} + \frac{1}{2} + \frac{1}{2}}

Combine the fractions

\frac{1}{2} + \frac{1}{2} + \frac{1}{2} : \frac{3}{2}

Apply rule

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

= \frac{1 + 1 + 1}{2}

Add the numbers:

1 + 1 + 1 = 3

= \frac{3}{2}

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

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

= 2^{1 + \frac{1}{2}}

Apply exponent rule:

x^{a + b} = x^{a} x^{b}

= 2^{1} \cdot 2^{\frac{1}{2}}

Refine

= 22

= 4 \cdot 22

Multiply the numbers:

4 \cdot 2 = 8

= 82

= (22)^{2} + 222 \cdot 22 + (22)^{2}

Apply Perfect Square Formula:

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

(22)^{2} + 222 \cdot 22 + (22)^{2} = (22 + 22)^{2}

= (22 + 22)^{2}

Apply radical rule:

(22 + 22)^{2} = 22 + 22

= 22 + 22

Apply radical rule:

a a = a

22 = 2

= 2 + 22

u_{1, 2} = \frac{- ( 2 - 2 2 ) \pm ( 2 + 2 2 )}{2 \cdot 4}

Separate the solutions

u_{1} = \frac{- ( 2 - 2 2 ) + 2 + 2 2}{2 \cdot 4}, u_{2} = \frac{- ( 2 - 2 2 ) - ( 2 + 2 2 )}{2 \cdot 4}

u = \frac{- ( 2 - 2 2 ) + 2 + 2 2}{2 \cdot 4} : \frac{2}{2}

\frac{- ( 2 - 2 2 ) + 2 + 2 2}{2 \cdot 4}

Multiply the numbers:

2 \cdot 4 = 8

= \frac{- ( 2 - 2 2 ) + 2 + 2 2}{8}

Expand

- (2 - 22) + 2 + 22 : 42

- (2 - 22) + 2 + 22

- (2 - 22) : - 2 + 22

- (2 - 22)

Distribute parentheses

= - (2) - (- 22)

Apply minus-plus rules

- (- a) = a, - (a) = - a

= - 2 + 22

= - 2 + 22 + 2 + 22

Simplify

- 2 + 22 + 2 + 22 : 42

- 2 + 22 + 2 + 22

Add similar elements:

22 + 22 = 42

= - 2 + 42 + 2

- 2 + 2 = 0

= 42

= 42

= \frac{4 2}{8}

Cancel the common factor:

4

= \frac{2}{2}

u = \frac{- ( 2 - 2 2 ) - ( 2 + 2 2 )}{2 \cdot 4} : - \frac{1}{2}

\frac{- ( 2 - 2 2 ) - ( 2 + 2 2 )}{2 \cdot 4}

Multiply the numbers:

2 \cdot 4 = 8

= \frac{- ( 2 - 2 2 ) - ( 2 + 2 2 )}{8}

Expand

- (2 - 22) - (2 + 22) : - 4

- (2 - 22) - (2 + 22)

- (2 - 22) : - 2 + 22

- (2 - 22)

Distribute parentheses

= - (2) - (- 22)

Apply minus-plus rules

- (- a) = a, - (a) = - a

= - 2 + 22

= - 2 + 22 - (2 + 22)

- (2 + 22) : - 2 - 22

- (2 + 22)

Distribute parentheses

= - (2) - (22)

Apply minus-plus rules

+ (- a) = - a

= - 2 - 22

= - 2 + 22 - 2 - 22

Simplify

- 2 + 22 - 2 - 22 : - 4

- 2 + 22 - 2 - 22

Add similar elements:

22 - 22 = 0

= - 2 - 2

Subtract the numbers:

- 2 - 2 = - 4

= - 4

= - 4

= \frac{- 4}{8}

Apply the fraction rule:

\frac{- a}{b} = - \frac{a}{b}

= - \frac{4}{8}

Cancel the common factor:

4

= - \frac{1}{2}

The solutions to the quadratic equation are:

u = \frac{2}{2}, u = - \frac{1}{2}

Substitute back

u = cos (x)

cos (x) = \frac{2}{2}, cos (x) = - \frac{1}{2}

cos (x) = \frac{2}{2}, cos (x) = - \frac{1}{2}

cos (x) = \frac{2}{2} : x = \frac{π}{4} + 2 πn, x = \frac{7 π}{4} + 2 πn

cos (x) = \frac{2}{2}

General solutions for

cos (x) = \frac{2}{2}

cos (x)

periodicity table with

2 πn

cycle:

x 0 \frac{π}{6} \frac{π}{4} \frac{π}{3} \frac{π}{2} \frac{2 π}{3} \frac{3 π}{4} \frac{5 π}{6} cos (x) 1 \frac{3}{2} \frac{2}{2} \frac{1}{2} 0 - \frac{1}{2} - \frac{2}{2} - \frac{3}{2} x π \frac{7 π}{6} \frac{5 π}{4} \frac{4 π}{3} \frac{3 π}{2} \frac{5 π}{3} \frac{7 π}{4} \frac{11 π}{6} cos (x) - 1 - \frac{3}{2} - \frac{2}{2} - \frac{1}{2} 0 \frac{1}{2} \frac{2}{2} \frac{3}{2}

x = \frac{π}{4} + 2 πn, x = \frac{7 π}{4} + 2 πn

x = \frac{π}{4} + 2 πn, x = \frac{7 π}{4} + 2 πn

cos (x) = - \frac{1}{2} : x = \frac{2 π}{3} + 2 πn, x = \frac{4 π}{3} + 2 πn

cos (x) = - \frac{1}{2}

General solutions for

cos (x) = - \frac{1}{2}

cos (x)

periodicity table with

2 πn

cycle:

x 0 \frac{π}{6} \frac{π}{4} \frac{π}{3} \frac{π}{2} \frac{2 π}{3} \frac{3 π}{4} \frac{5 π}{6} cos (x) 1 \frac{3}{2} \frac{2}{2} \frac{1}{2} 0 - \frac{1}{2} - \frac{2}{2} - \frac{3}{2} x π \frac{7 π}{6} \frac{5 π}{4} \frac{4 π}{3} \frac{3 π}{2} \frac{5 π}{3} \frac{7 π}{4} \frac{11 π}{6} cos (x) - 1 - \frac{3}{2} - \frac{2}{2} - \frac{1}{2} 0 \frac{1}{2} \frac{2}{2} \frac{3}{2}

x = \frac{2 π}{3} + 2 πn, x = \frac{4 π}{3} + 2 πn

x = \frac{2 π}{3} + 2 πn, x = \frac{4 π}{3} + 2 πn

Combine all the solutions

x = \frac{π}{4} + 2 πn, x = \frac{7 π}{4} + 2 πn, x = \frac{2 π}{3} + 2 πn, x = \frac{4 π}{3} + 2 πn

Graph

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