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Popular Trigonometry >

tan(x)<= cos(x)

Solution

tan (x) \leq cos (x)

Solution

2 πn \leq x \leq 0.66623 \dots + 2 πn or \frac{π}{2} + 2 πn < x \leq π - 0.66623 \dots + 2 πn or \frac{3 π}{2} + 2 πn < x \leq 2 π + 2 πn

Interval Notation

[2 πn, 0.66623 \dots + 2 πn] \cup (\frac{π}{2} + 2 πn, π - 0.66623 \dots + 2 πn] \cup (\frac{3 π}{2} + 2 πn, 2 π + 2 πn]

Decimal

2 πn \leq x \leq 0.66623 \dots + 2 πn or 1.57079 \dots + 2 πn < x \leq 2.47535 \dots + 2 πn or 4.71238 \dots + 2 πn < x \leq 6.28318 \dots + 2 πn

Solution steps

tan (x) \leq cos (x)

Move

cos (x)

to the left side

tan (x) \leq cos (x)

Subtract

cos (x)

from both sides

tan (x) - cos (x) \leq cos (x) - cos (x)

tan (x) - cos (x) \leq 0

tan (x) - cos (x) \leq 0

Periodicity of

tan (x) - cos (x) : 2 π

The compound periodicity of the sum of periodic functions is the least common multiplier of the periods

tan (x), cos (x)

Periodicity of

tan (x) : π

Periodicity of

tan (x)

π

= π

Periodicity of

cos (x) : 2 π

Periodicity of

cos (x)

2 π

= 2 π

Combine periods:

π, 2 π

= 2 π

Express with sin, cos

tan (x) - cos (x) \leq 0

Use the basic trigonometric identity:

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

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

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

Simplify

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

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

Convert element to fraction:

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

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

Since the denominators are equal, combine the fractions:

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

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

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

sin (x) - cos (x) cos (x)

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

cos (x) cos (x)

Apply exponent rule:

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

cos (x) cos (x) = cos^{1 + 1} (x)

= cos^{1 + 1} (x)

Add the numbers:

1 + 1 = 2

= cos^{2} (x)

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

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

\frac{sin ( x ) - cos ^{2} ( x )}{cos ( x )} \leq 0

Find the zeroes and undifined points of

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

for

0 \leq x < 2 π

To find the zeroes, set the inequality to zero

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

\frac{sin ( x ) - cos ^{2} ( x )}{cos ( x )} = 0, 0 \leq x < 2 π : x = 0.66623 \dots, x = π - 0.66623 \dots

\frac{sin ( x ) - cos ^{2} ( x )}{cos ( x )} = 0, 0 \leq x < 2 π

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

sin (x) - cos^{2} (x) = 0

Rewrite using trig identities

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

Use the Pythagorean identity:

cos^{2} (x) + sin^{2} (x) = 1

cos^{2} (x) = 1 - sin^{2} (x)

= - (1 - sin^{2} (x)) + sin (x)

- (1 - sin^{2} (x)) : - 1 + sin^{2} (x)

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

Distribute parentheses

= - (1) - (- sin^{2} (x))

Apply minus-plus rules

- (- a) = a, - (a) = - a

= - 1 + sin^{2} (x)

= - 1 + sin^{2} (x) + sin (x)

- 1 + sin (x) + sin^{2} (x) = 0

Solve by substitution

- 1 + sin (x) + sin^{2} (x) = 0

Let:

sin (x) = u

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

- 1 + u + u^{2} = 0 : u = \frac{- 1 + 5}{2}, u = \frac{- 1 - 5}{2}

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

Write in the standard form

a x^{2} + b x + c = 0

u^{2} + u - 1 = 0

Solve with the quadratic formula

u^{2} + u - 1 = 0

Quadratic Equation Formula:

For

a = 1, b = 1, c = - 1

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

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

1^{2} - 4 \cdot 1 \cdot (- 1) = 5

1^{2} - 4 \cdot 1 \cdot (- 1)

Apply rule

1^{a} = 1

1^{2} = 1

= 1 - 4 \cdot 1 \cdot (- 1)

Apply rule

- (- a) = a

= 1 + 4 \cdot 1 \cdot 1

Multiply the numbers:

4 \cdot 1 \cdot 1 = 4

= 1 + 4

Add the numbers:

1 + 4 = 5

= 5

u_{1, 2} = \frac{- 1 \pm 5}{2 \cdot 1}

Separate the solutions

u_{1} = \frac{- 1 + 5}{2 \cdot 1}, u_{2} = \frac{- 1 - 5}{2 \cdot 1}

u = \frac{- 1 + 5}{2 \cdot 1} : \frac{- 1 + 5}{2}

\frac{- 1 + 5}{2 \cdot 1}

Multiply the numbers:

2 \cdot 1 = 2

= \frac{- 1 + 5}{2}

u = \frac{- 1 - 5}{2 \cdot 1} : \frac{- 1 - 5}{2}

\frac{- 1 - 5}{2 \cdot 1}

Multiply the numbers:

2 \cdot 1 = 2

= \frac{- 1 - 5}{2}

The solutions to the quadratic equation are:

u = \frac{- 1 + 5}{2}, u = \frac{- 1 - 5}{2}

Substitute back

u = sin (x)

sin (x) = \frac{- 1 + 5}{2}, sin (x) = \frac{- 1 - 5}{2}

sin (x) = \frac{- 1 + 5}{2}, sin (x) = \frac{- 1 - 5}{2}

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

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

Apply trig inverse properties

sin (x) = \frac{- 1 + 5}{2}

General solutions for

sin (x) = \frac{- 1 + 5}{2}

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

x = arcsin (\frac{- 1 + 5}{2}) + 2 πn, x = π - arcsin (\frac{- 1 + 5}{2}) + 2 πn

x = arcsin (\frac{- 1 + 5}{2}) + 2 πn, x = π - arcsin (\frac{- 1 + 5}{2}) + 2 πn

Solutions for the range

0 \leq x < 2 π

x = arcsin (\frac{5 - 1}{2}), x = π - arcsin (\frac{5 - 1}{2})

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

No Solution

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

- 1 \leq sin (x) \leq 1

No Solution

Combine all the solutions

x = arcsin (\frac{5 - 1}{2}), x = π - arcsin (\frac{5 - 1}{2})

Show solutions in decimal form

x = 0.66623 \dots, x = π - 0.66623 \dots

Find the undefined points:

x = \frac{π}{2}, x = \frac{3 π}{2}

Find the zeros of the denominator

cos (x) = 0

General solutions for

cos (x) = 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}

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

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

Solutions for the range

0 \leq x < 2 π

x = \frac{π}{2}, x = \frac{3 π}{2}

0.66623 \dots, \frac{π}{2}, π - 0.66623 \dots, \frac{3 π}{2}

Identify the intervals

0 < x < 0.66623 \dots, 0.66623 \dots < x < \frac{π}{2}, \frac{π}{2} < x < π - 0.66623 \dots, π - 0.66623 \dots < x < \frac{3 π}{2}, \frac{3 π}{2} < x < 2 π

Summarize in a table:

sin (x) - cos^{2} (x) cos (x) \frac{s i n ( x ) - c o s ^{2} ( x )}{c o s ( x )} x = 0 - + - 0 < x < 0.66623 \dots - + - x = 0.66623 \dots 0 + 0 0.66623 \dots < x < \frac{π}{2} + + + x = \frac{π}{2} + 0 Undefined \frac{π}{2} < x < π - 0.66623 \dots + - - x = π - 0.66623 \dots 0 - 0 π - 0.66623 \dots < x < \frac{3 π}{2} - - + x = \frac{3 π}{2} - 0 Undefined \frac{3 π}{2} < x < 2 π - + - x = 2 π - + -

Identify the intervals that satisfy the required condition:

\leq 0

x = 0 or 0 < x < 0.66623 \dots or x = 0.66623 \dots or \frac{π}{2} < x < π - 0.66623 \dots or x = π - 0.66623 \dots or \frac{3 π}{2} < x < 2 π or x = 2 π

Merge Overlapping Intervals

0 \leq x \leq 0.66623 \dots or \frac{π}{2} < x \leq π - 0.66623 \dots or \frac{3 π}{2} < x < 2 π or x = 2 π