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Popular Trigonometry >

cos(x/2)+1/2 <0

Solution

cos (\frac{x}{2}) + \frac{1}{2} < 0

Solution

\frac{4 π}{3} + 4 πn < x < \frac{8 π}{3} + 4 πn

Interval Notation

(\frac{4 π}{3} + 4 πn, \frac{8 π}{3} + 4 πn)

Decimal

4.18879 \dots + 4 πn < x < 8.37758 \dots + 4 πn

Solution steps

cos (\frac{x}{2}) + \frac{1}{2} < 0

Move

\frac{1}{2}

to the right side

cos (\frac{x}{2}) + \frac{1}{2} < 0

Subtract

\frac{1}{2}

from both sides

cos (\frac{x}{2}) + \frac{1}{2} - \frac{1}{2} < 0 - \frac{1}{2}

Simplify

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

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

For

cos (x) < a

, if

- 1 < a \leq 1

then

arccos (a) + 2 πn < x < 2 π - arccos (a) + 2 πn

arccos (- \frac{1}{2}) + 2 πn < \frac{x}{2} < 2 π - arccos (- \frac{1}{2}) + 2 πn

a < u < b

then

a < u and u < b

arccos (- \frac{1}{2}) + 2 πn < \frac{x}{2} and \frac{x}{2} < 2 π - arccos (- \frac{1}{2}) + 2 πn

arccos (- \frac{1}{2}) + 2 πn < \frac{x}{2} : x > \frac{4 π}{3} + 4 πn

arccos (- \frac{1}{2}) + 2 πn < \frac{x}{2}

Switch sides

\frac{x}{2} > arccos (- \frac{1}{2}) + 2 πn

Simplify

arccos (- \frac{1}{2}) + 2 πn : \frac{2 π}{3} + 2 πn

arccos (- \frac{1}{2}) + 2 πn

Use the following trivial identity:

arccos (- \frac{1}{2}) = \frac{2 π}{3}

x - 1 - \frac{3}{2} - \frac{2}{2} - \frac{1}{2} 0 \frac{1}{2} \frac{2}{2} \frac{3}{2} 1 arccos (x) π \frac{5 π}{6} \frac{3 π}{4} \frac{2 π}{3} \frac{π}{2} \frac{π}{3} \frac{π}{4} \frac{π}{6} 0 arccos (x) 18 0^{\circ} 15 0^{\circ} 13 5^{\circ} 12 0^{\circ} 9 0^{\circ} 6 0^{\circ} 4 5^{\circ} 3 0^{\circ} 0^{\circ}

= \frac{2 π}{3} + 2 πn

\frac{x}{2} > \frac{2 π}{3} + 2 πn

Multiply both sides by

2

\frac{x}{2} > \frac{2 π}{3} + 2 πn

Multiply both sides by

2

\frac{2 x}{2} > 2 \cdot \frac{2 π}{3} + 2 \cdot 2 πn

Simplify

\frac{2 x}{2} > 2 \cdot \frac{2 π}{3} + 2 \cdot 2 πn

Simplify

\frac{2 x}{2} : x

\frac{2 x}{2}

Divide the numbers:

\frac{2}{2} = 1

= x

Simplify

2 \cdot \frac{2 π}{3} + 2 \cdot 2 πn : \frac{4 π}{3} + 4 πn

2 \cdot \frac{2 π}{3} + 2 \cdot 2 πn

2 \cdot \frac{2 π}{3} = \frac{4 π}{3}

2 \cdot \frac{2 π}{3}

Multiply fractions:

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

= \frac{2 π 2}{3}

Multiply the numbers:

2 \cdot 2 = 4

= \frac{4 π}{3}

2 \cdot 2 πn = 4 πn

2 \cdot 2 πn

Multiply the numbers:

2 \cdot 2 = 4

= 4 πn

= \frac{4 π}{3} + 4 πn

x > \frac{4 π}{3} + 4 πn

x > \frac{4 π}{3} + 4 πn

x > \frac{4 π}{3} + 4 πn

\frac{x}{2} < 2 π - arccos (- \frac{1}{2}) + 2 πn : x < \frac{8 π}{3} + 4 πn

\frac{x}{2} < 2 π - arccos (- \frac{1}{2}) + 2 πn

Simplify

2 π - arccos (- \frac{1}{2}) + 2 πn : 2 π - \frac{2 π}{3} + 2 πn

2 π - arccos (- \frac{1}{2}) + 2 πn

Use the following trivial identity:

arccos (- \frac{1}{2}) = \frac{2 π}{3}

x - 1 - \frac{3}{2} - \frac{2}{2} - \frac{1}{2} 0 \frac{1}{2} \frac{2}{2} \frac{3}{2} 1 arccos (x) π \frac{5 π}{6} \frac{3 π}{4} \frac{2 π}{3} \frac{π}{2} \frac{π}{3} \frac{π}{4} \frac{π}{6} 0 arccos (x) 18 0^{\circ} 15 0^{\circ} 13 5^{\circ} 12 0^{\circ} 9 0^{\circ} 6 0^{\circ} 4 5^{\circ} 3 0^{\circ} 0^{\circ}

= 2 π - \frac{2 π}{3} + 2 πn

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

Multiply both sides by

2

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

Multiply both sides by

2

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

Simplify

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

Simplify

\frac{2 x}{2} : x

\frac{2 x}{2}

Divide the numbers:

\frac{2}{2} = 1

= x

Simplify

2 \cdot 2 π - 2 \cdot \frac{2 π}{3} + 2 \cdot 2 πn : 4 π - \frac{4 π}{3} + 4 πn

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

2 \cdot 2 π = 4 π

2 \cdot 2 π

Multiply the numbers:

2 \cdot 2 = 4

= 4 π

2 \cdot \frac{2 π}{3} = \frac{4 π}{3}

2 \cdot \frac{2 π}{3}

Multiply fractions:

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

= \frac{2 π 2}{3}

Multiply the numbers:

2 \cdot 2 = 4

= \frac{4 π}{3}

2 \cdot 2 πn = 4 πn

2 \cdot 2 πn

Multiply the numbers:

2 \cdot 2 = 4

= 4 πn

= 4 π - \frac{4 π}{3} + 4 πn

x < 4 π - \frac{4 π}{3} + 4 πn

x < 4 π - \frac{4 π}{3} + 4 πn

Simplify

4 π - \frac{4 π}{3} : \frac{8 π}{3}

4 π - \frac{4 π}{3}

Convert element to fraction:

4 π = \frac{4 π 3}{3}

= \frac{4 π 3}{3} - \frac{4 π}{3}

Since the denominators are equal, combine the fractions:

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

= \frac{4 π 3 - 4 π}{3}

4 π 3 - 4 π = 8 π

4 π 3 - 4 π

Multiply the numbers:

4 \cdot 3 = 12

= 12 π - 4 π

Add similar elements:

12 π - 4 π = 8 π

= 8 π

= \frac{8 π}{3}

x < \frac{8 π}{3} + 4 πn

x < \frac{8 π}{3} + 4 πn

Combine the intervals

x > \frac{4 π}{3} + 4 πn and x < \frac{8 π}{3} + 4 πn

Merge Overlapping Intervals

\frac{4 π}{3} + 4 πn < x < \frac{8 π}{3} + 4 πn

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