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Popular Trigonometry >

solvefor x,f=(cos(2x))/(cos(x)-sin(x))

Solution

solvefor

Solution

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

Solution steps

f = \frac{cos ( 2 x )}{cos ( x ) - sin ( x )}

Switch sides

\frac{cos ( 2 x )}{cos ( x ) - sin ( x )} = f

Subtract

f

from both sides

\frac{cos ( 2 x )}{cos ( x ) - sin ( x )} - f = 0

Simplify

\frac{cos ( 2 x )}{cos ( x ) - sin ( x )} - f : \frac{cos ( 2 x ) - f ( cos ( x ) - sin ( x ) )}{cos ( x ) - sin ( x )}

\frac{cos ( 2 x )}{cos ( x ) - sin ( x )} - f

Convert element to fraction:

f = \frac{f ( cos ( x ) - sin ( x ) )}{cos ( x ) - sin ( x )}

= \frac{cos ( 2 x )}{cos ( x ) - sin ( x )} - \frac{f ( cos ( x ) - sin ( x ) )}{cos ( x ) - sin ( x )}

Since the denominators are equal, combine the fractions:

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

= \frac{cos ( 2 x ) - f ( cos ( x ) - sin ( x ) )}{cos ( x ) - sin ( x )}

\frac{cos ( 2 x ) - f ( cos ( x ) - sin ( x ) )}{cos ( x ) - sin ( x )} = 0

\frac{f ( x )}{g ( x )} = 0 \Rightarrow f (x) = 0

cos (2 x) - f (cos (x) - sin (x)) = 0

Rewrite using trig identities

cos (2 x) - (cos (x) - sin (x)) f

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

cos (2 x)

Use the Double Angle identity:

cos (2 x) = cos^{2} (x) - sin^{2} (x)

= cos^{2} (x) - sin^{2} (x)

Factor

cos^{2} (x) - sin^{2} (x) : (cos (x) + sin (x)) (cos (x) - sin (x))

cos^{2} (x) - sin^{2} (x)

Apply Difference of Two Squares Formula:

x^{2} - y^{2} = (x + y) (x - y)

cos^{2} (x) - sin^{2} (x) = (cos (x) + sin (x)) (cos (x) - sin (x))

= (cos (x) + sin (x)) (cos (x) - sin (x))

= (cos (x) + sin (x)) (cos (x) - sin (x))

= (cos (x) + sin (x)) (cos (x) - sin (x)) - f (cos (x) - sin (x))

(cos (x) + sin (x)) (cos (x) - sin (x)) - (cos (x) - sin (x)) f = 0

Factor

(cos (x) + sin (x)) (cos (x) - sin (x)) - (cos (x) - sin (x)) f : (cos (x) - sin (x)) (cos (x) + sin (x) - f)

(cos (x) + sin (x)) (cos (x) - sin (x)) - (cos (x) - sin (x)) f

Rewrite as

= (cos (x) - sin (x)) (cos (x) + sin (x)) - (cos (x) - sin (x)) f

Factor out common term

(cos (x) - sin (x))

= (cos (x) - sin (x)) (cos (x) + sin (x) - f)

(cos (x) - sin (x)) (cos (x) + sin (x) - f) = 0

Solving each part separately

cos (x) - sin (x) = 0 or cos (x) + sin (x) - f = 0

cos (x) - sin (x) = 0 : x = \frac{π}{4} + πn

cos (x) - sin (x) = 0

Rewrite using trig identities

cos (x) - sin (x) = 0

Divide both sides by

cos (x), cos (x) \neq = 0

\frac{cos ( x ) - sin ( x )}{cos ( x )} = \frac{0}{cos ( x )}

Simplify

1 - \frac{sin ( x )}{cos ( x )} = 0

Use the basic trigonometric identity:

\frac{sin ( x )}{cos ( x )} = tan (x)

1 - tan (x) = 0

1 - tan (x) = 0

Move

1

to the right side

1 - tan (x) = 0

Subtract

1

from both sides

1 - tan (x) - 1 = 0 - 1

Simplify

- tan (x) = - 1

- tan (x) = - 1

Divide both sides by

- 1

- tan (x) = - 1

Divide both sides by

- 1

\frac{- tan ( x )}{- 1} = \frac{- 1}{- 1}

Simplify

tan (x) = 1

tan (x) = 1

General solutions for

tan (x) = 1

tan (x)

periodicity table with

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} tan (x) 0 \frac{3}{3} 13 \pm \infty - 3 - 1 - \frac{3}{3}

x = \frac{π}{4} + πn

x = \frac{π}{4} + πn

cos (x) + sin (x) - f = 0 : x = arcsin (\frac{f}{2}) + 2 πn - \frac{π}{4}, x = π + arcsin (- \frac{f}{2}) + 2 πn - \frac{π}{4}

cos (x) + sin (x) - f = 0

Rewrite using trig identities

cos (x) + sin (x) - f

sin (x) + cos (x) = 2 sin (x + \frac{π}{4})

sin (x) + cos (x)

Rewrite as

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

Use the following trivial identity:

cos (\frac{π}{4}) = \frac{1}{2}

Use the following trivial identity:

sin (\frac{π}{4}) = \frac{1}{2}

= 2 (cos (\frac{π}{4}) sin (x) + sin (\frac{π}{4}) cos (x))

Use the Angle Sum identity:

sin (s + t) = sin (s) cos (t) + cos (s) sin (t)

= 2 sin (x + \frac{π}{4})

= - f + 2 sin (x + \frac{π}{4})

- f + 2 sin (x + \frac{π}{4}) = 0

Move

f

to the right side

- f + 2 sin (x + \frac{π}{4}) = 0

Add

f

to both sides

- f + 2 sin (x + \frac{π}{4}) + f = 0 + f

Simplify

2 sin (x + \frac{π}{4}) = f

2 sin (x + \frac{π}{4}) = f

Divide both sides by

2

2 sin (x + \frac{π}{4}) = f

Divide both sides by

2

\frac{2 sin ( x + \frac{π}{4} )}{2} = \frac{f}{2}

Simplify

\frac{2 sin ( x + \frac{π}{4} )}{2} = \frac{f}{2}

Simplify

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

\frac{2 sin ( x + \frac{π}{4} )}{2}

Cancel the common factor:

2

= sin (x + \frac{π}{4})

Simplify

\frac{f}{2} : \frac{2 f}{2}

\frac{f}{2}

Multiply by the conjugate

\frac{2}{2}

= \frac{f 2}{2 2}

22 = 2

22

Apply radical rule:

a a = a

22 = 2

= 2

= \frac{2 f}{2}

sin (x + \frac{π}{4}) = \frac{2 f}{2}

sin (x + \frac{π}{4}) = \frac{2 f}{2}

sin (x + \frac{π}{4}) = \frac{2 f}{2}

Apply trig inverse properties

sin (x + \frac{π}{4}) = \frac{2 f}{2}

General solutions for

sin (x + \frac{π}{4}) = \frac{2 f}{2}

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

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

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

Solve

x + \frac{π}{4} = arcsin (\frac{2 f}{2}) + 2 πn : x = arcsin (\frac{f}{2}) + 2 πn - \frac{π}{4}

x + \frac{π}{4} = arcsin (\frac{2 f}{2}) + 2 πn

Simplify

arcsin (\frac{2 f}{2}) + 2 πn : arcsin (\frac{f}{2}) + 2 πn

arcsin (\frac{2 f}{2}) + 2 πn

\frac{2 f}{2} = \frac{f}{2}

\frac{2 f}{2}

Apply radical rule:

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

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

Apply exponent rule:

\frac{x ^{a}}{x ^{b}} = \frac{1}{x ^{b - a}}

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

= \frac{f}{2 ^{1 - \frac{1}{2}}}

Subtract the numbers:

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

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

Apply radical rule:

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

= \frac{f}{2}

= arcsin (\frac{f}{2}) + 2 πn

x + \frac{π}{4} = arcsin (\frac{f}{2}) + 2 πn

Move

\frac{π}{4}

to the right side

x + \frac{π}{4} = arcsin (\frac{f}{2}) + 2 πn

Subtract

\frac{π}{4}

from both sides

x + \frac{π}{4} - \frac{π}{4} = arcsin (\frac{f}{2}) + 2 πn - \frac{π}{4}

Simplify

x = arcsin (\frac{f}{2}) + 2 πn - \frac{π}{4}

x = arcsin (\frac{f}{2}) + 2 πn - \frac{π}{4}

Solve

x + \frac{π}{4} = π + arcsin (- \frac{2 f}{2}) + 2 πn : x = π + arcsin (- \frac{f}{2}) + 2 πn - \frac{π}{4}

x + \frac{π}{4} = π + arcsin (- \frac{2 f}{2}) + 2 πn

Simplify

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

π + arcsin (- \frac{2 f}{2}) + 2 πn

\frac{2 f}{2} = \frac{f}{2}

\frac{2 f}{2}

Apply radical rule:

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

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

Apply exponent rule:

\frac{x ^{a}}{x ^{b}} = \frac{1}{x ^{b - a}}

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

= \frac{f}{2 ^{1 - \frac{1}{2}}}

Subtract the numbers:

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

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

Apply radical rule:

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

= \frac{f}{2}

= π + arcsin (- \frac{f}{2}) + 2 πn

x + \frac{π}{4} = π + arcsin (- \frac{f}{2}) + 2 πn

Move

\frac{π}{4}

to the right side

x + \frac{π}{4} = π + arcsin (- \frac{f}{2}) + 2 πn

Subtract

\frac{π}{4}

from both sides

x + \frac{π}{4} - \frac{π}{4} = π + arcsin (- \frac{f}{2}) + 2 πn - \frac{π}{4}

Simplify

x = π + arcsin (- \frac{f}{2}) + 2 πn - \frac{π}{4}

x = π + arcsin (- \frac{f}{2}) + 2 πn - \frac{π}{4}

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

Combine all the solutions

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

Since the equation is undefined for:

\frac{π}{4} + πn

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

Graph

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