What is the general solution for sec(x)+sin^2(x)+cos^2(x)=tan^2(x) ?

The general solution for sec(x)+sin^2(x)+cos^2(x)=tan^2(x) is x=pi+2pin,x= pi/3+2pin,x=(5pi)/3+2pin

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sec(x)+sin^2(x)+cos^2(x)=tan^2(x)

Solution

sec (x) + sin^{2} (x) + cos^{2} (x) = tan^{2} (x)

Solution

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

Degrees

x = 18 0^{\circ} + 36 0^{\circ} n, x = 6 0^{\circ} + 36 0^{\circ} n, x = 30 0^{\circ} + 36 0^{\circ} n

Solution steps

sec (x) + sin^{2} (x) + cos^{2} (x) = tan^{2} (x)

Subtract

tan^{2} (x)

from both sides

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

Express with sin, cos

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

Use the basic trigonometric identity:

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

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

Use the basic trigonometric identity:

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

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

Simplify

cos^{2} (x) + \frac{1}{cos ( x )} + sin^{2} (x) - (\frac{sin ( x )}{cos ( x )})^{2} : \frac{cos ^{4} ( x ) + cos ( x ) + sin ^{2} ( x ) cos ^{2} ( x ) - sin ^{2} ( x )}{cos ^{2} ( x )}

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

Apply exponent rule:

(\frac{a}{b})^{c} = \frac{a ^{c}}{b ^{c}}

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

Convert element to fraction:

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

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

Least Common Multiplier of

1, cos (x), 1, cos^{2} (x) : cos^{2} (x)

1, cos (x), 1, cos^{2} (x)

Lowest Common Multiplier (LCM)

Least Common Multiplier of

1, 1 : 1

1, 1

Least Common Multiplier (LCM)

Prime factorization of

1

Prime factorization of

1

Multiply each factor the greatest number of times it occurs in either

1

1

= 1

Multiply the numbers:

1 = 1

= 1

Compute an expression comprised of factors that appear in at least one of the factored expressions

= cos^{2} (x)

Adjust Fractions based on the LCM

Multiply each numerator by the same amount needed to multiply its

corresponding denominator to turn it into the LCM

cos^{2} (x)

For

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

multiply the denominator and numerator by

cos^{2} (x)

\frac{cos ^{2} ( x )}{1} = \frac{cos ^{2} ( x ) cos ^{2} ( x )}{1 \cdot cos ^{2} ( x )} = \frac{cos ^{4} ( x )}{cos ^{2} ( x )}

For

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

multiply the denominator and numerator by

cos (x)

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

For

\frac{sin ^{2} ( x )}{1} :

multiply the denominator and numerator by

cos^{2} (x)

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

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

Since the denominators are equal, combine the fractions:

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

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

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

\frac{cos ( x ) + cos ^{4} ( x ) - sin ^{2} ( x ) + cos ^{2} ( x ) sin ^{2} ( x )}{cos ^{2} ( x )} = 0

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

cos (x) + cos^{4} (x) - sin^{2} (x) + cos^{2} (x) sin^{2} (x) = 0

Factor

cos (x) + cos^{4} (x) - sin^{2} (x) + cos^{2} (x) sin^{2} (x) : (1 + cos (x)) (cos (x) (cos^{2} (x) + 1 - cos (x)) + sin^{2} (x) (- 1 + cos (x)))

cos (x) + cos^{4} (x) - sin^{2} (x) + cos^{2} (x) sin^{2} (x)

Factor

cos (x) + cos^{4} (x) : cos (x) (cos (x) + 1) (cos^{2} (x) - cos (x) + 1)

cos (x) + cos^{4} (x)

Apply exponent rule:

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

cos^{4} (x) = cos (x) cos^{3} (x)

= cos (x) + cos (x) cos^{3} (x)

Factor out common term

cos (x)

= cos (x) (1 + cos^{3} (x))

Factor

cos^{3} (x) + 1 : (cos (x) + 1) (cos^{2} (x) - cos (x) + 1)

1 + cos^{3} (x)

Rewrite

1

1^{3}

= cos^{3} (x) + 1^{3}

Apply Sum of Cubes Formula:

x^{3} + y^{3} = (x + y) (x^{2} - x y + y^{2})

cos^{3} (x) + 1^{3} = (cos (x) + 1) (cos^{2} (x) - cos (x) + 1)

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

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

Factor

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

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

Factor out common term

sin^{2} (x)

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

Factor

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

- 1 + cos^{2} (x)

Rewrite

1

1^{2}

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

Apply Difference of Two Squares Formula:

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

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

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

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

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

Factor out common term

(1 + cos (x))

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

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

Solving each part separately

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

1 + cos (x) = 0 : x = π + 2 πn

1 + cos (x) = 0

Move

1

to the right side

1 + cos (x) = 0

Subtract

1

from both sides

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

Simplify

cos (x) = - 1

cos (x) = - 1

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

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

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

Rewrite using trig identities

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

Use the Pythagorean identity:

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

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

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

Simplify

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

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

Simplify

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

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

Group like terms

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

Add the numbers:

1 + 1 = 2

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

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

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

Expand

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

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

Apply the distributive law:

a (b + c) = ab + a c

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

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

Apply minus-plus rules

+ (- a) = - a

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

Multiply:

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

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

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

Expand

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

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

Distribute parentheses

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

Apply minus-plus rules

+ (- a) = - a

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

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

Add similar elements:

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

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

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

Use the Pythagorean identity:

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

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

= 2 cos (x) - 1

2 cos (x) - 1 = 0

Move

1

to the right side

2 cos (x) - 1 = 0

Add

1

to both sides

2 cos (x) - 1 + 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

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

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

General solutions for

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

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

Combine all the solutions

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

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Frequently Asked Questions (FAQ)

What is the general solution for sec(x)+sin^2(x)+cos^2(x)=tan^2(x) ?
The general solution for sec(x)+sin^2(x)+cos^2(x)=tan^2(x) is x=pi+2pin,x= pi/3+2pin,x=(5pi)/3+2pin