What is the general solution for cos^2(3x)=cos(9x^2) ?

The general solution for cos^2(3x)=cos(9x^2) is x=(pi+4pin)/6 ,x=(3pi+4pin)/6 ,x=(2pin)/3

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cos^2(3x)=cos(9x^2)

Solution

cos^{2} (3 x) = cos (9 x^{2})

Solution

x = \frac{π + 4 πn}{6}, x = \frac{3 π + 4 πn}{6}, x = \frac{2 πn}{3}

Degrees

x = 3 0^{\circ} + 12 0^{\circ} n, x = 9 0^{\circ} + 12 0^{\circ} n, x = 0^{\circ} + 12 0^{\circ} n

Solution steps

cos^{2} (3 x) = cos (9 x^{2})

Subtract

cos (9 x^{2})

from both sides

cos^{2} (3 x) - cos (9 x^{2}) = 0

Let:

u = 3 x

cos^{2} (u) - cos (\frac{u ^{2}}{u}) = 0

Factor

cos^{2} (u) - cos (\frac{u ^{2}}{u}) : cos (u) (cos (u) - 1)

cos^{2} (u) - cos (\frac{u ^{2}}{u})

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

\frac{u ^{2}}{u}

Cancel the common factor:

u

= u

= cos^{2} (u) - cos (u)

Apply exponent rule:

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

cos^{2} (u) = cos (u) cos (u)

= cos (u) cos (u) - cos (u)

Factor out common term

cos (u)

= cos (u) (cos (u) - 1)

cos (u) (cos (u) - 1) = 0

Solving each part separately

cos (u) = 0 or cos (u) - 1 = 0

cos (u) = 0 : u = \frac{π}{2} + 2 πn, u = \frac{3 π}{2} + 2 πn

cos (u) = 0

General solutions for

cos (u) = 0

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}

u = \frac{π}{2} + 2 πn, u = \frac{3 π}{2} + 2 πn

u = \frac{π}{2} + 2 πn, u = \frac{3 π}{2} + 2 πn

cos (u) - 1 = 0 : u = 2 πn

cos (u) - 1 = 0

Move

1

to the right side

cos (u) - 1 = 0

Add

1

to both sides

cos (u) - 1 + 1 = 0 + 1

Simplify

cos (u) = 1

cos (u) = 1

General solutions for

cos (u) = 1

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}

u = 0 + 2 πn

u = 0 + 2 πn

Solve

u = 0 + 2 πn : u = 2 πn

u = 0 + 2 πn

0 + 2 πn = 2 πn

u = 2 πn

u = 2 πn

Combine all the solutions

u = \frac{π}{2} + 2 πn, u = \frac{3 π}{2} + 2 πn, u = 2 πn

Substitute back

u = 3 x

3 x = \frac{π}{2} + 2 πn : x = \frac{π + 4 πn}{6}

3 x = \frac{π}{2} + 2 πn

Divide both sides by

3

3 x = \frac{π}{2} + 2 πn

Divide both sides by

3

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

Simplify

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

Simplify

\frac{3 x}{3} : x

\frac{3 x}{3}

Divide the numbers:

\frac{3}{3} = 1

= x

Simplify

\frac{\frac{π}{2}}{3} + \frac{2 πn}{3} : \frac{π + 4 πn}{6}

\frac{\frac{π}{2}}{3} + \frac{2 πn}{3}

Apply rule

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

= \frac{\frac{π}{2} + 2 πn}{3}

Join

\frac{π}{2} + 2 πn : \frac{π + 4 πn}{2}

\frac{π}{2} + 2 πn

Convert element to fraction:

2 πn = \frac{2 πn 2}{2}

= \frac{π}{2} + \frac{2 πn \cdot 2}{2}

Since the denominators are equal, combine the fractions:

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

= \frac{π + 2 πn \cdot 2}{2}

Multiply the numbers:

2 \cdot 2 = 4

= \frac{π + 4 πn}{2}

= \frac{\frac{π + 4 πn}{2}}{3}

Apply the fraction rule:

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

= \frac{π + 4 πn}{2 \cdot 3}

Multiply the numbers:

2 \cdot 3 = 6

= \frac{π + 4 πn}{6}

x = \frac{π + 4 πn}{6}

x = \frac{π + 4 πn}{6}

x = \frac{π + 4 πn}{6}

3 x = \frac{3 π}{2} + 2 πn : x = \frac{3 π + 4 πn}{6}

3 x = \frac{3 π}{2} + 2 πn

Divide both sides by

3

3 x = \frac{3 π}{2} + 2 πn

Divide both sides by

3

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

Simplify

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

Simplify

\frac{3 x}{3} : x

\frac{3 x}{3}

Divide the numbers:

\frac{3}{3} = 1

= x

Simplify

\frac{\frac{3 π}{2}}{3} + \frac{2 πn}{3} : \frac{3 π + 4 πn}{6}

\frac{\frac{3 π}{2}}{3} + \frac{2 πn}{3}

Apply rule

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

= \frac{\frac{3 π}{2} + 2 πn}{3}

Join

\frac{3 π}{2} + 2 πn : \frac{3 π + 4 πn}{2}

\frac{3 π}{2} + 2 πn

Convert element to fraction:

2 πn = \frac{2 πn 2}{2}

= \frac{3 π}{2} + \frac{2 πn \cdot 2}{2}

Since the denominators are equal, combine the fractions:

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

= \frac{3 π + 2 πn \cdot 2}{2}

Multiply the numbers:

2 \cdot 2 = 4

= \frac{3 π + 4 πn}{2}

= \frac{\frac{3 π + 4 πn}{2}}{3}

Apply the fraction rule:

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

= \frac{3 π + 4 πn}{2 \cdot 3}

Multiply the numbers:

2 \cdot 3 = 6

= \frac{3 π + 4 πn}{6}

x = \frac{3 π + 4 πn}{6}

x = \frac{3 π + 4 πn}{6}

x = \frac{3 π + 4 πn}{6}

3 x = 2 πn : x = \frac{2 πn}{3}

3 x = 2 πn

Divide both sides by

3

3 x = 2 πn

Divide both sides by

3

\frac{3 x}{3} = \frac{2 πn}{3}

Simplify

x = \frac{2 πn}{3}

x = \frac{2 πn}{3}

x = \frac{π + 4 πn}{6}, x = \frac{3 π + 4 πn}{6}, x = \frac{2 πn}{3}

Since the equation is undefined for:

\frac{π + 4 πn}{6}, \frac{3 π + 4 πn}{6}, \frac{2 πn}{3}

x = \frac{π + 4 πn}{6}, x = \frac{3 π + 4 πn}{6}, x = \frac{2 πn}{3}

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Frequently Asked Questions (FAQ)

What is the general solution for cos^2(3x)=cos(9x^2) ?
The general solution for cos^2(3x)=cos(9x^2) is x=(pi+4pin)/6 ,x=(3pi+4pin)/6 ,x=(2pin)/3