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Popular Trigonometry >

(-tan^2(x))/2 =cos^2(x)-1

Solution

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

Solution

x = 2 πn, x = π + 2 πn, x = \frac{3 π}{4} + 2 πn, x = \frac{5 π}{4} + 2 πn, x = \frac{π}{4} + 2 πn, x = \frac{7 π}{4} + 2 πn

Degrees

x = 0^{\circ} + 36 0^{\circ} n, x = 18 0^{\circ} + 36 0^{\circ} n, x = 13 5^{\circ} + 36 0^{\circ} n, x = 22 5^{\circ} + 36 0^{\circ} n, x = 4 5^{\circ} + 36 0^{\circ} n, x = 31 5^{\circ} + 36 0^{\circ} n

Solution steps

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

Subtract

cos^{2} (x) - 1

from both sides

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

Simplify

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

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

Convert element to fraction:

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

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

Since the denominators are equal, combine the fractions:

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

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

Multiply the numbers:

1 \cdot 2 = 2

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

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

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

- tan^{2} (x) - 2 cos^{2} (x) + 2 = 0

Rewrite using trig identities

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

Use the Pythagorean identity:

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

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

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

Simplify

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

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

Expand

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

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

Apply the distributive law:

a (b - c) = ab - a c

a = - 2, b = 1, c = sin^{2} (x)

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

Apply minus-plus rules

- (- a) = a

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

Multiply the numbers:

2 \cdot 1 = 2

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

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

Simplify

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

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

Group like terms

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

2 - 2 = 0

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

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

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

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

Factor

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

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

Rewrite

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

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

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

Apply radical rule:

a = (a)^{2}

2 = (2)^{2}

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

Apply exponent rule:

a^{m} b^{m} = (ab)^{m}

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

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

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

Apply Difference of Two Squares Formula:

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

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

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

(2 sin (x) + tan (x)) (2 sin (x) - tan (x)) = 0

Solving each part separately

2 sin (x) + tan (x) = 0 or 2 sin (x) - tan (x) = 0

2 sin (x) + tan (x) = 0 : x = 2 πn, x = π + 2 πn, x = \frac{3 π}{4} + 2 πn, x = \frac{5 π}{4} + 2 πn

2 sin (x) + tan (x) = 0

Express with sin, cos

tan (x) + sin (x) 2

Use the basic trigonometric identity:

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

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

Simplify

\frac{sin ( x )}{cos ( x )} + sin (x) 2 : \frac{sin ( x ) + 2 sin ( x ) cos ( x )}{cos ( x )}

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

Convert element to fraction:

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

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

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

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

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

sin (x) + cos (x) sin (x) 2 = 0

Factor

sin (x) + cos (x) sin (x) 2 : sin (x) (1 + 2 cos (x))

sin (x) + cos (x) sin (x) 2

Factor out common term

sin (x)

= sin (x) (1 + cos (x) 2)

sin (x) (1 + 2 cos (x)) = 0

Solving each part separately

sin (x) = 0 or 1 + 2 cos (x) = 0

sin (x) = 0 : x = 2 πn, x = π + 2 πn

sin (x) = 0

General solutions for

sin (x) = 0

sin (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} sin (x) 0 \frac{1}{2} \frac{2}{2} \frac{3}{2} 1 \frac{3}{2} \frac{2}{2} \frac{1}{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} sin (x) 0 - \frac{1}{2} - \frac{2}{2} - \frac{3}{2} - 1 - \frac{3}{2} - \frac{2}{2} - \frac{1}{2}

x = 0 + 2 πn, x = π + 2 πn

x = 0 + 2 πn, x = π + 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, x = π + 2 πn

1 + 2 cos (x) = 0 : x = \frac{3 π}{4} + 2 πn, x = \frac{5 π}{4} + 2 πn

1 + 2 cos (x) = 0

Move

1

to the right side

1 + 2 cos (x) = 0

Subtract

1

from both sides

1 + 2 cos (x) - 1 = 0 - 1

Simplify

2 cos (x) = - 1

2 cos (x) = - 1

Divide both sides by

2

2 cos (x) = - 1

Divide both sides by

2

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

Simplify

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

Simplify

\frac{2 cos ( x )}{2} : cos (x)

\frac{2 cos ( x )}{2}

Cancel the common factor:

2

= cos (x)

Simplify

\frac{- 1}{2} : - \frac{2}{2}

\frac{- 1}{2}

Apply the fraction rule:

\frac{- a}{b} = - \frac{a}{b}

= - \frac{1}{2}

Rationalize

- \frac{1}{2} : - \frac{2}{2}

- \frac{1}{2}

Multiply by the conjugate

\frac{2}{2}

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

1 \cdot 2 = 2

22 = 2

22

Apply radical rule:

a a = a

22 = 2

= 2

= - \frac{2}{2}

= - \frac{2}{2}

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

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

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

General solutions for

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

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{3 π}{4} + 2 πn, x = \frac{5 π}{4} + 2 πn

x = \frac{3 π}{4} + 2 πn, x = \frac{5 π}{4} + 2 πn

Combine all the solutions

x = 2 πn, x = π + 2 πn, x = \frac{3 π}{4} + 2 πn, x = \frac{5 π}{4} + 2 πn

2 sin (x) - tan (x) = 0 : x = 2 πn, x = π + 2 πn, x = \frac{π}{4} + 2 πn, x = \frac{7 π}{4} + 2 πn

2 sin (x) - tan (x) = 0

Express with sin, cos

- tan (x) + sin (x) 2

Use the basic trigonometric identity:

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

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

Simplify

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

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

Convert element to fraction:

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

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

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

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

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

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

Factor

- sin (x) + cos (x) sin (x) 2 : sin (x) (- 1 + 2 cos (x))

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

Factor out common term

sin (x)

= sin (x) (- 1 + cos (x) 2)

sin (x) (- 1 + 2 cos (x)) = 0

Solving each part separately

sin (x) = 0 or - 1 + 2 cos (x) = 0

sin (x) = 0 : x = 2 πn, x = π + 2 πn

sin (x) = 0

General solutions for

sin (x) = 0

sin (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} sin (x) 0 \frac{1}{2} \frac{2}{2} \frac{3}{2} 1 \frac{3}{2} \frac{2}{2} \frac{1}{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} sin (x) 0 - \frac{1}{2} - \frac{2}{2} - \frac{3}{2} - 1 - \frac{3}{2} - \frac{2}{2} - \frac{1}{2}

x = 0 + 2 πn, x = π + 2 πn

x = 0 + 2 πn, x = π + 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, x = π + 2 πn

- 1 + 2 cos (x) = 0 : x = \frac{π}{4} + 2 πn, x = \frac{7 π}{4} + 2 πn

- 1 + 2 cos (x) = 0

Move

1

to the right side

- 1 + 2 cos (x) = 0

Add

1

to both sides

- 1 + 2 cos (x) + 1 = 0 + 1

Simplify

2 cos (x) = 1

2 cos (x) = 1

Divide both sides by

2

2 cos (x) = 1

Divide both sides by

2

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

Simplify

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

Simplify

\frac{2 cos ( x )}{2} : cos (x)

\frac{2 cos ( x )}{2}

Cancel the common factor:

2

= cos (x)

Simplify

\frac{1}{2} : \frac{2}{2}

\frac{1}{2}

Multiply by the conjugate

\frac{2}{2}

= \frac{1 \cdot 2}{2 2}

1 \cdot 2 = 2

22 = 2

22

Apply radical rule:

a a = a

22 = 2

= 2

= \frac{2}{2}

cos (x) = \frac{2}{2}

cos (x) = \frac{2}{2}

cos (x) = \frac{2}{2}

General solutions for

cos (x) = \frac{2}{2}

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}{4} + 2 πn, x = \frac{7 π}{4} + 2 πn

x = \frac{π}{4} + 2 πn, x = \frac{7 π}{4} + 2 πn

Combine all the solutions

x = 2 πn, x = π + 2 πn, x = \frac{π}{4} + 2 πn, x = \frac{7 π}{4} + 2 πn

Combine all the solutions

x = 2 πn, x = π + 2 πn, x = \frac{3 π}{4} + 2 πn, x = \frac{5 π}{4} + 2 πn, x = \frac{π}{4} + 2 πn, x = \frac{7 π}{4} + 2 πn

Graph

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