What is the general solution for 2cos^2(x)+2cos(x)-3=0 ?

The general solution for 2cos^2(x)+2cos(x)-3=0 is x=0.60434…+2pin,x=2pi-0.60434…+2pin

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2cos^2(x)+2cos(x)-3=0

Solution

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

Solution

x = 0.60434 \dots + 2 πn, x = 2 π - 0.60434 \dots + 2 πn

Degrees

x = 34.62629 \dots^{\circ} + 36 0^{\circ} n, x = 325.37370 \dots^{\circ} + 36 0^{\circ} n

Solution steps

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

Solve by substitution

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

Let:

cos (x) = u

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

2 u^{2} + 2 u - 3 = 0 : u = \frac{- 1 + 7}{2}, u = - \frac{1 + 7}{2}

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

Solve with the quadratic formula

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

Quadratic Equation Formula:

For

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

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

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

2^{2} - 4 \cdot 2 (- 3) = 27

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

Apply rule

- (- a) = a

= 2^{2} + 4 \cdot 2 \cdot 3

Multiply the numbers:

4 \cdot 2 \cdot 3 = 24

= 2^{2} + 24

2^{2} = 4

= 4 + 24

Add the numbers:

4 + 24 = 28

= 28

Prime factorization of

28 : 2^{2} \cdot 7

28

28

divides by

228 = 14 \cdot 2

= 2 \cdot 14

14

divides by

214 = 7 \cdot 2

= 2 \cdot 2 \cdot 7

2, 7

are all prime numbers, therefore no further factorization is possible

= 2 \cdot 2 \cdot 7

= 2^{2} \cdot 7

= 2^{2} \cdot 7

Apply radical rule:

= 7 2^{2}

Apply radical rule:

2^{2} = 2

= 27

u_{1, 2} = \frac{- 2 \pm 2 7}{2 \cdot 2}

Separate the solutions

u_{1} = \frac{- 2 + 2 7}{2 \cdot 2}, u_{2} = \frac{- 2 - 2 7}{2 \cdot 2}

u = \frac{- 2 + 2 7}{2 \cdot 2} : \frac{- 1 + 7}{2}

\frac{- 2 + 2 7}{2 \cdot 2}

Multiply the numbers:

2 \cdot 2 = 4

= \frac{- 2 + 2 7}{4}

Factor

- 2 + 27 : 2 (- 1 + 7)

- 2 + 27

Rewrite as

= - 2 \cdot 1 + 27

Factor out common term

2

= 2 (- 1 + 7)

= \frac{2 ( - 1 + 7 )}{4}

Cancel the common factor:

2

= \frac{- 1 + 7}{2}

u = \frac{- 2 - 2 7}{2 \cdot 2} : - \frac{1 + 7}{2}

\frac{- 2 - 2 7}{2 \cdot 2}

Multiply the numbers:

2 \cdot 2 = 4

= \frac{- 2 - 2 7}{4}

Factor

- 2 - 27 : - 2 (1 + 7)

- 2 - 27

Rewrite as

= - 2 \cdot 1 - 27

Factor out common term

2

= - 2 (1 + 7)

= - \frac{2 ( 1 + 7 )}{4}

Cancel the common factor:

2

= - \frac{1 + 7}{2}

The solutions to the quadratic equation are:

u = \frac{- 1 + 7}{2}, u = - \frac{1 + 7}{2}

Substitute back

u = cos (x)

cos (x) = \frac{- 1 + 7}{2}, cos (x) = - \frac{1 + 7}{2}

cos (x) = \frac{- 1 + 7}{2}, cos (x) = - \frac{1 + 7}{2}

cos (x) = \frac{- 1 + 7}{2} : x = arccos (\frac{- 1 + 7}{2}) + 2 πn, x = 2 π - arccos (\frac{- 1 + 7}{2}) + 2 πn

cos (x) = \frac{- 1 + 7}{2}

Apply trig inverse properties

cos (x) = \frac{- 1 + 7}{2}

General solutions for

cos (x) = \frac{- 1 + 7}{2}

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

x = arccos (\frac{- 1 + 7}{2}) + 2 πn, x = 2 π - arccos (\frac{- 1 + 7}{2}) + 2 πn

x = arccos (\frac{- 1 + 7}{2}) + 2 πn, x = 2 π - arccos (\frac{- 1 + 7}{2}) + 2 πn

cos (x) = - \frac{1 + 7}{2} :

No Solution

cos (x) = - \frac{1 + 7}{2}

- 1 \leq cos (x) \leq 1

No Solution

Combine all the solutions

x = arccos (\frac{- 1 + 7}{2}) + 2 πn, x = 2 π - arccos (\frac{- 1 + 7}{2}) + 2 πn

Show solutions in decimal form

x = 0.60434 \dots + 2 πn, x = 2 π - 0.60434 \dots + 2 πn

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Frequently Asked Questions (FAQ)

What is the general solution for 2cos^2(x)+2cos(x)-3=0 ?
The general solution for 2cos^2(x)+2cos(x)-3=0 is x=0.60434…+2pin,x=2pi-0.60434…+2pin