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Popular >

5sin(4x)=4cos(2x)

Solution

5 sin (4 x) = 4 cos (2 x)

Solution

x = \frac{π + 4 πn}{4}, x = \frac{3 π + 4 πn}{4}, x = \frac{0.41151 \dots + 2 πn}{2}, x = \frac{π - 0.41151 \dots + 2 πn}{2}

Degrees

x = 4 5^{\circ} + 18 0^{\circ} n, x = 13 5^{\circ} + 18 0^{\circ} n, x = 11.78908 \dots^{\circ} + 18 0^{\circ} n, x = 78.21091 \dots^{\circ} + 18 0^{\circ} n

Solution steps

5 sin (4 x) = 4 cos (2 x)

Subtract

4 cos (2 x)

from both sides

5 sin (4 x) - 4 cos (2 x) = 0

Let:

u = 2 x

5 sin (2 u) - 4 cos (u) = 0

Rewrite using trig identities

- 4 cos (u) + 5 sin (2 u)

Use the Double Angle identity:

sin (2 x) = 2 sin (x) cos (x)

= - 4 cos (u) + 5 \cdot 2 sin (u) cos (u)

Simplify

= - 4 cos (u) + 10 sin (u) cos (u)

- 4 cos (u) + 10 cos (u) sin (u) = 0

Factor

- 4 cos (u) + 10 cos (u) sin (u) : 2 cos (u) (5 sin (u) - 2)

- 4 cos (u) + 10 cos (u) sin (u)

Rewrite

10

5 \cdot 2

Rewrite

- 4

2 \cdot 2

= 2 \cdot 2 cos (u) + 5 \cdot 2 sin (u) cos (u)

Factor out common term

2 cos (u)

= 2 cos (u) (- 2 + 5 sin (u))

2 cos (u) (5 sin (u) - 2) = 0

Solving each part separately

cos (u) = 0 or 5 sin (u) - 2 = 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

5 sin (u) - 2 = 0 : u = arcsin (\frac{2}{5}) + 2 πn, u = π - arcsin (\frac{2}{5}) + 2 πn

5 sin (u) - 2 = 0

Move

2

to the right side

5 sin (u) - 2 = 0

Add

2

to both sides

5 sin (u) - 2 + 2 = 0 + 2

Simplify

5 sin (u) = 2

5 sin (u) = 2

Divide both sides by

5

5 sin (u) = 2

Divide both sides by

5

\frac{5 sin ( u )}{5} = \frac{2}{5}

Simplify

sin (u) = \frac{2}{5}

sin (u) = \frac{2}{5}

Apply trig inverse properties

sin (u) = \frac{2}{5}

General solutions for

sin (u) = \frac{2}{5}

sin (x) = a \Rightarrow x = arcsin (a) + 2 πn, x = π - arcsin (a) + 2 πn

u = arcsin (\frac{2}{5}) + 2 πn, u = π - arcsin (\frac{2}{5}) + 2 πn

u = arcsin (\frac{2}{5}) + 2 πn, u = π - arcsin (\frac{2}{5}) + 2 πn

Combine all the solutions

u = \frac{π}{2} + 2 πn, u = \frac{3 π}{2} + 2 πn, u = arcsin (\frac{2}{5}) + 2 πn, u = π - arcsin (\frac{2}{5}) + 2 πn

Substitute back

u = 2 x

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

2 x = \frac{π}{2} + 2 πn

Divide both sides by

2

2 x = \frac{π}{2} + 2 πn

Divide both sides by

2

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

Simplify

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

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

Apply rule

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

= \frac{\frac{π}{2} + 2 πn}{2}

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}}{2}

Apply the fraction rule:

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

= \frac{π + 4 πn}{2 \cdot 2}

Multiply the numbers:

2 \cdot 2 = 4

= \frac{π + 4 πn}{4}

x = \frac{π + 4 πn}{4}

x = \frac{π + 4 πn}{4}

x = \frac{π + 4 πn}{4}

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

2 x = \frac{3 π}{2} + 2 πn

Divide both sides by

2

2 x = \frac{3 π}{2} + 2 πn

Divide both sides by

2

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

Simplify

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

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

Apply rule

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

= \frac{\frac{3 π}{2} + 2 πn}{2}

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}}{2}

Apply the fraction rule:

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

= \frac{3 π + 4 πn}{2 \cdot 2}

Multiply the numbers:

2 \cdot 2 = 4

= \frac{3 π + 4 πn}{4}

x = \frac{3 π + 4 πn}{4}

x = \frac{3 π + 4 πn}{4}

x = \frac{3 π + 4 πn}{4}

2 x = arcsin (\frac{2}{5}) + 2 πn : x = \frac{arcsin ( \frac{2}{5} ) + 2 πn}{2}

2 x = arcsin (\frac{2}{5}) + 2 πn

Divide both sides by

2

2 x = arcsin (\frac{2}{5}) + 2 πn

Divide both sides by

2

\frac{2 x}{2} = \frac{arcsin ( \frac{2}{5} )}{2} + \frac{2 πn}{2}

Simplify

\frac{2 x}{2} = \frac{arcsin ( \frac{2}{5} )}{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{arcsin ( \frac{2}{5} )}{2} + \frac{2 πn}{2} : \frac{arcsin ( \frac{2}{5} ) + 2 πn}{2}

\frac{arcsin ( \frac{2}{5} )}{2} + \frac{2 πn}{2}

Apply rule

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

= \frac{arcsin ( \frac{2}{5} ) + 2 πn}{2}

x = \frac{arcsin ( \frac{2}{5} ) + 2 πn}{2}

x = \frac{arcsin ( \frac{2}{5} ) + 2 πn}{2}

x = \frac{arcsin ( \frac{2}{5} ) + 2 πn}{2}

2 x = π - arcsin (\frac{2}{5}) + 2 πn : x = \frac{π - arcsin ( \frac{2}{5} ) + 2 πn}{2}

2 x = π - arcsin (\frac{2}{5}) + 2 πn

Divide both sides by

2

2 x = π - arcsin (\frac{2}{5}) + 2 πn

Divide both sides by

2

\frac{2 x}{2} = \frac{π}{2} - \frac{arcsin ( \frac{2}{5} )}{2} + \frac{2 πn}{2}

Simplify

\frac{2 x}{2} = \frac{π}{2} - \frac{arcsin ( \frac{2}{5} )}{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{π}{2} - \frac{arcsin ( \frac{2}{5} )}{2} + \frac{2 πn}{2} : \frac{π - arcsin ( \frac{2}{5} ) + 2 πn}{2}

\frac{π}{2} - \frac{arcsin ( \frac{2}{5} )}{2} + \frac{2 πn}{2}

Apply rule

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

= \frac{π - arcsin ( \frac{2}{5} ) + 2 πn}{2}

x = \frac{π - arcsin ( \frac{2}{5} ) + 2 πn}{2}

x = \frac{π - arcsin ( \frac{2}{5} ) + 2 πn}{2}

x = \frac{π - arcsin ( \frac{2}{5} ) + 2 πn}{2}

x = \frac{π + 4 πn}{4}, x = \frac{3 π + 4 πn}{4}, x = \frac{arcsin ( \frac{2}{5} ) + 2 πn}{2}, x = \frac{π - arcsin ( \frac{2}{5} ) + 2 πn}{2}

Show solutions in decimal form

x = \frac{π + 4 πn}{4}, x = \frac{3 π + 4 πn}{4}, x = \frac{0.41151 \dots + 2 πn}{2}, x = \frac{π - 0.41151 \dots + 2 πn}{2}

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