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Popular Trigonometry >

cos(2x)-cos(6x)-sin(4x)=0,0<= x<= pi

Solution

cos (2 x) - cos (6 x) - sin (4 x) = 0, 0 \leq x \leq π

Solution

x = 0, x = \frac{π}{4}, x = \frac{π}{2}, x = \frac{3 π}{4}, x = π, x = \frac{π}{12}, x = \frac{5 π}{12}

Degrees

x = 0^{\circ}, x = 4 5^{\circ}, x = 9 0^{\circ}, x = 13 5^{\circ}, x = 18 0^{\circ}, x = 1 5^{\circ}, x = 7 5^{\circ}

Solution steps

cos (2 x) - cos (6 x) - sin (4 x) = 0, 0 \leq x \leq π

Rewrite using trig identities

cos (2 x) - cos (6 x) - sin (4 x)

Use the Sum to Product identity:

cos (s) - cos (t) = - 2 sin (\frac{s + t}{2}) sin (\frac{s - t}{2})

= - sin (4 x) - 2 sin (\frac{2 x + 6 x}{2}) sin (\frac{2 x - 6 x}{2})

Simplify

- sin (4 x) - 2 sin (\frac{2 x + 6 x}{2}) sin (\frac{2 x - 6 x}{2}) : - sin (4 x) + 2 sin (2 x) sin (4 x)

- sin (4 x) - 2 sin (\frac{2 x + 6 x}{2}) sin (\frac{2 x - 6 x}{2})

2 sin (\frac{2 x + 6 x}{2}) sin (\frac{2 x - 6 x}{2}) = - 2 sin (2 x) sin (4 x)

2 sin (\frac{2 x + 6 x}{2}) sin (\frac{2 x - 6 x}{2})

\frac{2 x + 6 x}{2} = 4 x

\frac{2 x + 6 x}{2}

Add similar elements:

2 x + 6 x = 8 x

= \frac{8 x}{2}

Divide the numbers:

\frac{8}{2} = 4

= 4 x

= 2 sin (4 x) sin (\frac{2 x - 6 x}{2})

\frac{2 x - 6 x}{2} = - 2 x

\frac{2 x - 6 x}{2}

Add similar elements:

2 x - 6 x = - 4 x

= \frac{- 4 x}{2}

Apply the fraction rule:

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

= - \frac{4 x}{2}

Divide the numbers:

\frac{4}{2} = 2

= - 2 x

= 2 sin (4 x) sin (- 2 x)

Use the negative angle identity:

sin (- x) = - sin (x)

= 2 (- sin (2 x)) sin (4 x)

= - sin (4 x) - (- 2 sin (2 x) sin (4 x))

Apply rule

- (- a) = a

= - sin (4 x) + 2 sin (2 x) sin (4 x)

= - sin (4 x) + 2 sin (2 x) sin (4 x)

- sin (4 x) + 2 sin (2 x) sin (4 x) = 0

Factor

- sin (4 x) + 2 sin (2 x) sin (4 x) : sin (4 x) (2 sin (2 x) - 1)

- sin (4 x) + 2 sin (2 x) sin (4 x)

Factor out common term

sin (4 x)

= sin (4 x) (- 1 + 2 sin (2 x))

sin (4 x) (2 sin (2 x) - 1) = 0

Solving each part separately

sin (4 x) = 0 or 2 sin (2 x) - 1 = 0

sin (4 x) = 0, 0 \leq x \leq π : x = 0, x = \frac{π}{4}, x = \frac{π}{2}, x = \frac{3 π}{4}, x = π

sin (4 x) = 0, 0 \leq x \leq π

General solutions for

sin (4 x) = 0

sin (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} sin (x) 0 \frac{1}{2} \frac{2}{2} \frac{3}{2} 1 \frac{3}{2} \frac{2}{2} \frac{1}{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} sin (x) 0 - \frac{1}{2} - \frac{2}{2} - \frac{3}{2} - 1 - \frac{3}{2} - \frac{2}{2} - \frac{1}{2}

4 x = 0 + 2 πn, 4 x = π + 2 πn

4 x = 0 + 2 πn, 4 x = π + 2 πn

Solve

4 x = 0 + 2 πn : x = \frac{πn}{2}

4 x = 0 + 2 πn

0 + 2 πn = 2 πn

4 x = 2 πn

Divide both sides by

4

4 x = 2 πn

Divide both sides by

4

\frac{4 x}{4} = \frac{2 πn}{4}

Simplify

x = \frac{πn}{2}

x = \frac{πn}{2}

Solve

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

4 x = π + 2 πn

Divide both sides by

4

4 x = π + 2 πn

Divide both sides by

4

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

Simplify

x = \frac{π}{4} + \frac{πn}{2}

x = \frac{π}{4} + \frac{πn}{2}

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

Solutions for the range

0 \leq x \leq π

x = 0, x = \frac{π}{4}, x = \frac{π}{2}, x = \frac{3 π}{4}, x = π

2 sin (2 x) - 1 = 0, 0 \leq x \leq π : x = \frac{π}{12}, x = \frac{5 π}{12}

2 sin (2 x) - 1 = 0, 0 \leq x \leq π

Move

1

to the right side

2 sin (2 x) - 1 = 0

Add

1

to both sides

2 sin (2 x) - 1 + 1 = 0 + 1

Simplify

2 sin (2 x) = 1

2 sin (2 x) = 1

Divide both sides by

2

2 sin (2 x) = 1

Divide both sides by

2

\frac{2 sin ( 2 x )}{2} = \frac{1}{2}

Simplify

sin (2 x) = \frac{1}{2}

sin (2 x) = \frac{1}{2}

General solutions for

sin (2 x) = \frac{1}{2}

sin (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} sin (x) 0 \frac{1}{2} \frac{2}{2} \frac{3}{2} 1 \frac{3}{2} \frac{2}{2} \frac{1}{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} sin (x) 0 - \frac{1}{2} - \frac{2}{2} - \frac{3}{2} - 1 - \frac{3}{2} - \frac{2}{2} - \frac{1}{2}

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

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

Solve

2 x = \frac{π}{6} + 2 πn : x = \frac{π}{12} + πn

2 x = \frac{π}{6} + 2 πn

Divide both sides by

2

2 x = \frac{π}{6} + 2 πn

Divide both sides by

2

\frac{2 x}{2} = \frac{\frac{π}{6}}{2} + \frac{2 πn}{2}

Simplify

\frac{2 x}{2} = \frac{\frac{π}{6}}{2} + \frac{2 πn}{2}

Simplify

\frac{2 x}{2} : x

\frac{2 x}{2}

Divide the numbers:

\frac{2}{2} = 1

= x

Simplify

\frac{\frac{π}{6}}{2} + \frac{2 πn}{2} : \frac{π}{12} + πn

\frac{\frac{π}{6}}{2} + \frac{2 πn}{2}

\frac{\frac{π}{6}}{2} = \frac{π}{12}

\frac{\frac{π}{6}}{2}

Apply the fraction rule:

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

= \frac{π}{6 \cdot 2}

Multiply the numbers:

6 \cdot 2 = 12

= \frac{π}{12}

\frac{2 πn}{2} = πn

\frac{2 πn}{2}

Divide the numbers:

\frac{2}{2} = 1

= πn

= \frac{π}{12} + πn

x = \frac{π}{12} + πn

x = \frac{π}{12} + πn

x = \frac{π}{12} + πn

Solve

2 x = \frac{5 π}{6} + 2 πn : x = \frac{5 π}{12} + πn

2 x = \frac{5 π}{6} + 2 πn

Divide both sides by

2

2 x = \frac{5 π}{6} + 2 πn

Divide both sides by

2

\frac{2 x}{2} = \frac{\frac{5 π}{6}}{2} + \frac{2 πn}{2}

Simplify

\frac{2 x}{2} = \frac{\frac{5 π}{6}}{2} + \frac{2 πn}{2}

Simplify

\frac{2 x}{2} : x

\frac{2 x}{2}

Divide the numbers:

\frac{2}{2} = 1

= x

Simplify

\frac{\frac{5 π}{6}}{2} + \frac{2 πn}{2} : \frac{5 π}{12} + πn

\frac{\frac{5 π}{6}}{2} + \frac{2 πn}{2}

\frac{\frac{5 π}{6}}{2} = \frac{5 π}{12}

\frac{\frac{5 π}{6}}{2}

Apply the fraction rule:

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

= \frac{5 π}{6 \cdot 2}

Multiply the numbers:

6 \cdot 2 = 12

= \frac{5 π}{12}

\frac{2 πn}{2} = πn

\frac{2 πn}{2}

Divide the numbers:

\frac{2}{2} = 1

= πn

= \frac{5 π}{12} + πn

x = \frac{5 π}{12} + πn

x = \frac{5 π}{12} + πn

x = \frac{5 π}{12} + πn

x = \frac{π}{12} + πn, x = \frac{5 π}{12} + πn

Solutions for the range

0 \leq x \leq π

x = \frac{π}{12}, x = \frac{5 π}{12}

Combine all the solutions

x = 0, x = \frac{π}{4}, x = \frac{π}{2}, x = \frac{3 π}{4}, x = π, x = \frac{π}{12}, x = \frac{5 π}{12}

Graph

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