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sec(x)<= cos(x)

Solution

sec (x) \leq cos (x)

Solution

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

Interval Notation

(\frac{π}{2} + 2 πn, \frac{3 π}{2} + 2 πn)

Decimal Notation

1.57079 \dots + 2 πn < x < 4.71238 \dots + 2 πn

Solution steps

sec (x) \leq cos (x)

Move

cos (x)

to the left side

sec (x) \leq cos (x)

Subtract

cos (x)

from both sides

sec (x) - cos (x) \leq cos (x) - cos (x)

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

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

Periodicity of

sec (x) - cos (x) : 2 π

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

sec (x), cos (x)

Periodicity of

sec (x) : 2 π

Periodicity of

sec (x)

2 π

= 2 π

Periodicity of

cos (x) : 2 π

Periodicity of

cos (x)

2 π

= 2 π

Combine periods:

2 π, 2 π

= 2 π

Express with sin, cos

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

Use the basic trigonometric identity:

sec (x) = \frac{1}{cos ( x )}

\frac{1}{cos ( x )} - cos (x) \leq 0

\frac{1}{cos ( x )} - cos (x) \leq 0

Simplify

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

\frac{1}{cos ( x )} - cos (x)

Convert element to fraction:

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

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

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

1 - 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)

= 1 - cos^{2} (x)

= \frac{1 - cos ^{2} ( x )}{cos ( x )}

\frac{1 - cos ^{2} ( x )}{cos ( x )} \leq 0

Find the zeroes and undifined points of

\frac{1 - cos ^{2} ( x )}{cos ( x )}

for

0 \leq x < 2 π

To find the zeroes, set the inequality to zero

\frac{1 - cos ^{2} ( x )}{cos ( x )} = 0

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

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

Solve by substitution

\frac{1 - cos ^{2} ( x )}{cos ( x )} = 0

Let:

cos (x) = u

\frac{1 - u ^{2}}{u} = 0

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

\frac{1 - u ^{2}}{u} = 0

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

1 - u^{2} = 0

Solve

1 - u^{2} = 0 : u = 1, u = - 1

1 - u^{2} = 0

Move

1

to the right side

1 - u^{2} = 0

Subtract

1

from both sides

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

Simplify

- u^{2} = - 1

- u^{2} = - 1

Divide both sides by

- 1

- u^{2} = - 1

Divide both sides by

- 1

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

Simplify

u^{2} = 1

u^{2} = 1

For

x^{2} = f (a)

the solutions are

x = f (a), - f (a)

u = 1, u = - 1

1 = 1

1

Apply rule

1 = 1

= 1

- 1 = - 1

- 1

Apply rule

1 = 1

= - 1

u = 1, u = - 1

u = 1, u = - 1

Verify Solutions

Find undefined (singularity) points:

u = 0

Take the denominator(s) of

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

and compare to zero

u = 0

The following points are undefined

u = 0

Combine undefined points with solutions:

u = 1, u = - 1

Substitute back

u = cos (x)

cos (x) = 1, cos (x) = - 1

cos (x) = 1, cos (x) = - 1

cos (x) = 1, 0 \leq x < 2 π : x = 0

cos (x) = 1, 0 \leq x < 2 π

General solutions for

cos (x) = 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}

x = 0 + 2 πn

x = 0 + 2 πn

Solve

x = 0 + 2 πn : x = 2 πn

x = 0 + 2 πn

0 + 2 πn = 2 πn

x = 2 πn

x = 2 πn

Solutions for the range

0 \leq x < 2 π

x = 0

cos (x) = - 1, 0 \leq x < 2 π : x = π

cos (x) = - 1, 0 \leq x < 2 π

General solutions for

cos (x) = - 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}

x = π + 2 πn

x = π + 2 πn

Solutions for the range

0 \leq x < 2 π

x = π

Combine all the solutions

x = 0, x = π

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, \frac{π}{2}, π, \frac{3 π}{2}

Identify the intervals

0 < x < \frac{π}{2}, \frac{π}{2} < x < π, π < x < \frac{3 π}{2}, \frac{3 π}{2} < x < 2 π

Summarize in a table:

1 - cos^{2} (x) cos (x) \frac{1 - c o s ^{2} ( x )}{c o s ( x )} x = 0 0 + 0 0 < x < \frac{π}{2} + + + x = \frac{π}{2} + 0 Undefined \frac{π}{2} < x < π + - - x = π 0 - 0 π < x < \frac{3 π}{2} + - - x = \frac{3 π}{2} + 0 Undefined \frac{3 π}{2} < x < 2 π + + + x = 2 π 0 + 0

Identify the intervals that satisfy the required condition:

\leq 0

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

Merge Overlapping Intervals

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

The union of two intervals is the set of numbers which are in either interval

x = 0

\frac{π}{2} < x < π

x = 0 or \frac{π}{2} < x < π

The union of two intervals is the set of numbers which are in either interval

x = 0 or \frac{π}{2} < x < π

x = π

x = 0 or \frac{π}{2} < x \leq π

The union of two intervals is the set of numbers which are in either interval

x = 0 or \frac{π}{2} < x \leq π

π < x < \frac{3 π}{2}

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

The union of two intervals is the set of numbers which are in either interval

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

x = 2 π

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

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

Apply the periodicity of

sec (x) - cos (x)

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

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