What is the general solution for cot(x)=sin^2(x) ?

The general solution for cot(x)=sin^2(x) is x=0.97202…+2pin,x=pi+0.97202…+2pin

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cot(x)=sin^2(x)

Solution

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

Solution

x = 0.97202 \dots + 2 πn, x = π + 0.97202 \dots + 2 πn

Degrees

x = 55.69319 \dots^{\circ} + 36 0^{\circ} n, x = 235.69319 \dots^{\circ} + 36 0^{\circ} n

Solution steps

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

Subtract

sin^{2} (x)

from both sides

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

Express with sin, cos

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

Simplify

\frac{cos ( x )}{sin ( x )} - sin^{2} (x) : \frac{cos ( x ) - sin ^{3} ( x )}{sin ( x )}

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

Convert element to fraction:

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

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

Since the denominators are equal, combine the fractions:

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

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

cos (x) - sin^{2} (x) sin (x) = cos (x) - sin^{3} (x)

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

sin^{2} (x) sin (x) = sin^{3} (x)

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

Apply exponent rule:

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

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

= sin^{2 + 1} (x)

Add the numbers:

2 + 1 = 3

= sin^{3} (x)

= cos (x) - sin^{3} (x)

= \frac{cos ( x ) - sin ^{3} ( x )}{sin ( x )}

\frac{cos ( x ) - sin ^{3} ( x )}{sin ( x )} = 0

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

cos (x) - sin^{3} (x) = 0

Add

sin^{3} (x)

to both sides

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

Square both sides

cos^{2} (x) = (sin^{3} (x))^{2}

Subtract

(sin^{3} (x))^{2}

from both sides

cos^{2} (x) - sin^{6} (x) = 0

Rewrite using trig identities

cos^{2} (x) - sin^{6} (x)

Use the Pythagorean identity:

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

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

= 1 - sin^{2} (x) - sin^{6} (x)

1 - sin^{2} (x) - sin^{6} (x) = 0

Solve by substitution

1 - sin^{2} (x) - sin^{6} (x) = 0

Let:

sin (x) = u

1 - u^{2} - u^{6} = 0

1 - u^{2} - u^{6} = 0 : u = 0.68232 \dots, u = - 0.68232 \dots

1 - u^{2} - u^{6} = 0

Write in the standard form

a_{n} x^{n} + \dots + a_{1} x + a_{0} = 0

- u^{6} - u^{2} + 1 = 0

Rewrite the equation with

v = u^{2}

and

v^{3} = u^{6}

- v^{3} - v + 1 = 0

Solve

- v^{3} - v + 1 = 0 : v \approx 0.68232 \dots

- v^{3} - v + 1 = 0

Find one solution for

- v^{3} - v + 1 = 0

using Newton-Raphson:

v \approx 0.68232 \dots

- v^{3} - v + 1 = 0

Newton-Raphson Approximation Definition

f (v) = - v^{3} - v + 1

Find

f^{^{'}} (v) : - 3 v^{2} - 1

\frac{d}{d v} (- v^{3} - v + 1)

Apply the Sum/Difference Rule:

(f \pm g)^{'} = f^{'} \pm g^{'}

= - \frac{d}{d v} (v^{3}) - \frac{d v}{d v} + \frac{d}{d v} (1)

\frac{d}{d v} (v^{3}) = 3 v^{2}

\frac{d}{d v} (v^{3})

Apply the Power Rule:

\frac{d}{d x} (x^{a}) = a \cdot x^{a - 1}

= 3 v^{3 - 1}

Simplify

= 3 v^{2}

\frac{d v}{d v} = 1

\frac{d v}{d v}

Apply the common derivative:

\frac{d v}{d v} = 1

= 1

\frac{d}{d v} (1) = 0

\frac{d}{d v} (1)

Derivative of a constant:

\frac{d}{d x} (a) = 0

= 0

= - 3 v^{2} - 1 + 0

Simplify

= - 3 v^{2} - 1

Let

v_{0} = 1

Compute

v_{n + 1}

until

Δ v_{n + 1} < 0.000001

v_{1} = 0.75 : Δ v_{1} = 0.25

f (v_{0}) = - 1^{3} - 1 + 1 = - 1

f^{^{'}} (v_{0}) = - 3 \cdot 1^{2} - 1 = - 4

v_{1} = 0.75

Δ v_{1} = ∣ 0.75 - 1 ∣ = 0.25

Δ v_{1} = 0.25

v_{2} = 0.68604 \dots : Δ v_{2} = 0.06395 \dots

f (v_{1}) = - 0.7 5^{3} - 0.75 + 1 = - 0.171875

f^{^{'}} (v_{1}) = - 3 \cdot 0.7 5^{2} - 1 = - 2.6875

v_{2} = 0.68604 \dots

Δ v_{2} = ∣ 0.68604 \dots - 0.75 ∣ = 0.06395 \dots

Δ v_{2} = 0.06395 \dots

v_{3} = 0.68233 \dots : Δ v_{3} = 0.00370 \dots

f (v_{2}) = - 0.68604 \dots^{3} - 0.68604 \dots + 1 = - 0.00894 \dots

f^{^{'}} (v_{2}) = - 3 \cdot 0.68604 \dots^{2} - 1 = - 2.41197 \dots

v_{3} = 0.68233 \dots

Δ v_{3} = ∣ 0.68233 \dots - 0.68604 \dots ∣ = 0.00370 \dots

Δ v_{3} = 0.00370 \dots

v_{4} = 0.68232 \dots : Δ v_{4} = 0.00001 \dots

f (v_{3}) = - 0.68233 \dots^{3} - 0.68233 \dots + 1 = - 0.00002 \dots

f^{^{'}} (v_{3}) = - 3 \cdot 0.68233 \dots^{2} - 1 = - 2.39676 \dots

v_{4} = 0.68232 \dots

Δ v_{4} = ∣ 0.68232 \dots - 0.68233 \dots ∣ = 0.00001 \dots

Δ v_{4} = 0.00001 \dots

v_{5} = 0.68232 \dots : Δ v_{5} = 1.18493 E - 10

f (v_{4}) = - 0.68232 \dots^{3} - 0.68232 \dots + 1 = - 2.83995 E - 10

f^{^{'}} (v_{4}) = - 3 \cdot 0.68232 \dots^{2} - 1 = - 2.39671 \dots

v_{5} = 0.68232 \dots

Δ v_{5} = ∣ 0.68232 \dots - 0.68232 \dots ∣ = 1.18493 E - 10

Δ v_{5} = 1.18493 E - 10

v \approx 0.68232 \dots

Apply long division:

\frac{- v ^{3} - v + 1}{v - 0.68232 \dots} = - v^{2} - 0.68232 \dots v - 1.46557 \dots

- v^{2} - 0.68232 \dots v - 1.46557 \dots \approx 0

Find one solution for

- v^{2} - 0.68232 \dots v - 1.46557 \dots = 0

using Newton-Raphson:

No Solution for

v \in R

- v^{2} - 0.68232 \dots v - 1.46557 \dots = 0

Newton-Raphson Approximation Definition

f (v) = - v^{2} - 0.68232 \dots v - 1.46557 \dots

Find

f^{^{'}} (v) : - 2 v - 0.68232 \dots

\frac{d}{d v} (- v^{2} - 0.68232 \dots v - 1.46557 \dots)

Apply the Sum/Difference Rule:

(f \pm g)^{'} = f^{'} \pm g^{'}

= - \frac{d}{d v} (v^{2}) - \frac{d}{d v} (0.68232 \dots v) - \frac{d}{d v} (1.46557 \dots)

\frac{d}{d v} (v^{2}) = 2 v

\frac{d}{d v} (v^{2})

Apply the Power Rule:

\frac{d}{d x} (x^{a}) = a \cdot x^{a - 1}

= 2 v^{2 - 1}

Simplify

= 2 v

\frac{d}{d v} (0.68232 \dots v) = 0.68232 \dots

\frac{d}{d v} (0.68232 \dots v)

Take the constant out:

(a \cdot f)^{'} = a \cdot f^{'}

= 0.68232 \dots \frac{d v}{d v}

Apply the common derivative:

\frac{d v}{d v} = 1

= 0.68232 \dots \cdot 1

Simplify

= 0.68232 \dots

\frac{d}{d v} (1.46557 \dots) = 0

\frac{d}{d v} (1.46557 \dots)

Derivative of a constant:

\frac{d}{d x} (a) = 0

= 0

= - 2 v - 0.68232 \dots - 0

Simplify

= - 2 v - 0.68232 \dots

Let

v_{0} = - 2

Compute

v_{n + 1}

until

Δ v_{n + 1} < 0.000001

v_{1} = - 0.76391 \dots : Δ v_{1} = 1.23608 \dots

f (v_{0}) = - (- 2)^{2} - 0.68232 \dots (- 2) - 1.46557 \dots = - 4.10091 \dots

f^{^{'}} (v_{0}) = - 2 (- 2) - 0.68232 \dots = 3.31767 \dots

v_{1} = - 0.76391 \dots

Δ v_{1} = ∣ - 0.76391 \dots - (- 2) ∣ = 1.23608 \dots

Δ v_{1} = 1.23608 \dots

v_{2} = 1.04316 \dots : Δ v_{2} = 1.80707 \dots

f (v_{1}) = - (- 0.76391 \dots)^{2} - 0.68232 \dots (- 0.76391 \dots) - 1.46557 \dots = - 1.52789 \dots

f^{^{'}} (v_{1}) = - 2 (- 0.76391 \dots) - 0.68232 \dots = 0.84550 \dots

v_{2} = 1.04316 \dots

Δ v_{2} = ∣ 1.04316 \dots - (- 0.76391 \dots) ∣ = 1.80707 \dots

Δ v_{2} = 1.80707 \dots

v_{3} = - 0.13630 \dots : Δ v_{3} = 1.17946 \dots

f (v_{2}) = - 1.04316 \dots^{2} - 0.68232 \dots \cdot 1.04316 \dots - 1.46557 \dots = - 3.26553 \dots

f^{^{'}} (v_{2}) = - 2 \cdot 1.04316 \dots - 0.68232 \dots = - 2.76865 \dots

v_{3} = - 0.13630 \dots

Δ v_{3} = ∣ - 0.13630 \dots - 1.04316 \dots ∣ = 1.17946 \dots

Δ v_{3} = 1.17946 \dots

v_{4} = - 3.53171 \dots : Δ v_{4} = 3.39540 \dots

f (v_{3}) = - (- 0.13630 \dots)^{2} - 0.68232 \dots (- 0.13630 \dots) - 1.46557 \dots = - 1.39114 \dots

f^{^{'}} (v_{3}) = - 2 (- 0.13630 \dots) - 0.68232 \dots = - 0.40971 \dots

v_{4} = - 3.53171 \dots

Δ v_{4} = ∣ - 3.53171 \dots - (- 0.13630 \dots) ∣ = 3.39540 \dots

Δ v_{4} = 3.39540 \dots

v_{5} = - 1.72500 \dots : Δ v_{5} = 1.80670 \dots

f (v_{4}) = - (- 3.53171 \dots)^{2} - 0.68232 \dots (- 3.53171 \dots) - 1.46557 \dots = - 11.52876 \dots

f^{^{'}} (v_{4}) = - 2 (- 3.53171 \dots) - 0.68232 \dots = 6.38109 \dots

v_{5} = - 1.72500 \dots

Δ v_{5} = ∣ - 1.72500 \dots - (- 3.53171 \dots) ∣ = 1.80670 \dots

Δ v_{5} = 1.80670 \dots

v_{6} = - 0.54560 \dots : Δ v_{6} = 1.17939 \dots

f (v_{5}) = - (- 1.72500 \dots)^{2} - 0.68232 \dots (- 1.72500 \dots) - 1.46557 \dots = - 3.26419 \dots

f^{^{'}} (v_{5}) = - 2 (- 1.72500 \dots) - 0.68232 \dots = 2.76767 \dots

v_{6} = - 0.54560 \dots

Δ v_{6} = ∣ - 0.54560 \dots - (- 1.72500 \dots) ∣ = 1.17939 \dots

Δ v_{6} = 1.17939 \dots

v_{7} = 2.85625 \dots : Δ v_{7} = 3.40185 \dots

f (v_{6}) = - (- 0.54560 \dots)^{2} - 0.68232 \dots (- 0.54560 \dots) - 1.46557 \dots = - 1.39097 \dots

f^{^{'}} (v_{6}) = - 2 (- 0.54560 \dots) - 0.68232 \dots = 0.40888 \dots

v_{7} = 2.85625 \dots

Δ v_{7} = ∣ 2.85625 \dots - (- 0.54560 \dots) ∣ = 3.40185 \dots

Δ v_{7} = 3.40185 \dots

v_{8} = 1.04656 \dots : Δ v_{8} = 1.80968 \dots

f (v_{7}) = - 2.85625 \dots^{2} - 0.68232 \dots \cdot 2.85625 \dots - 1.46557 \dots = - 11.57264 \dots

f^{^{'}} (v_{7}) = - 2 \cdot 2.85625 \dots - 0.68232 \dots = - 6.39483 \dots

v_{8} = 1.04656 \dots

Δ v_{8} = ∣ 1.04656 \dots - 2.85625 \dots ∣ = 1.80968 \dots

Δ v_{8} = 1.80968 \dots

v_{9} = - 0.13340 \dots : Δ v_{9} = 1.17997 \dots

f (v_{8}) = - 1.04656 \dots^{2} - 0.68232 \dots \cdot 1.04656 \dots - 1.46557 \dots = - 3.27496 \dots

f^{^{'}} (v_{8}) = - 2 \cdot 1.04656 \dots - 0.68232 \dots = - 2.77545 \dots

v_{9} = - 0.13340 \dots

Δ v_{9} = ∣ - 0.13340 \dots - 1.04656 \dots ∣ = 1.17997 \dots

Δ v_{9} = 1.17997 \dots

v_{10} = - 3.48434 \dots : Δ v_{10} = 3.35093 \dots

f (v_{9}) = - (- 0.13340 \dots)^{2} - 0.68232 \dots (- 0.13340 \dots) - 1.46557 \dots = - 1.39234 \dots

f^{^{'}} (v_{9}) = - 2 (- 0.13340 \dots) - 0.68232 \dots = - 0.41550 \dots

v_{10} = - 3.48434 \dots

Δ v_{10} = ∣ - 3.48434 \dots - (- 0.13340 \dots) ∣ = 3.35093 \dots

Δ v_{10} = 3.35093 \dots

Cannot find solution

The solution is

v \approx 0.68232 \dots

v \approx 0.68232 \dots

Substitute back

v = u^{2},

solve for

u

Solve

u^{2} = 0.68232 \dots : u = 0.68232 \dots, u = - 0.68232 \dots

u^{2} = 0.68232 \dots

For

x^{2} = f (a)

the solutions are

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

u = 0.68232 \dots, u = - 0.68232 \dots

The solutions are

u = 0.68232 \dots, u = - 0.68232 \dots

Substitute back

u = sin (x)

sin (x) = 0.68232 \dots, sin (x) = - 0.68232 \dots

sin (x) = 0.68232 \dots, sin (x) = - 0.68232 \dots

sin (x) = 0.68232 \dots : x = arcsin (0.68232 \dots) + 2 πn, x = π - arcsin (0.68232 \dots) + 2 πn

sin (x) = 0.68232 \dots

Apply trig inverse properties

sin (x) = 0.68232 \dots

General solutions for

sin (x) = 0.68232 \dots

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

x = arcsin (0.68232 \dots) + 2 πn, x = π - arcsin (0.68232 \dots) + 2 πn

x = arcsin (0.68232 \dots) + 2 πn, x = π - arcsin (0.68232 \dots) + 2 πn

sin (x) = - 0.68232 \dots : x = arcsin (- 0.68232 \dots) + 2 πn, x = π + arcsin (0.68232 \dots) + 2 πn

sin (x) = - 0.68232 \dots

Apply trig inverse properties

sin (x) = - 0.68232 \dots

General solutions for

sin (x) = - 0.68232 \dots

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

x = arcsin (- 0.68232 \dots) + 2 πn, x = π + arcsin (0.68232 \dots) + 2 πn

x = arcsin (- 0.68232 \dots) + 2 πn, x = π + arcsin (0.68232 \dots) + 2 πn

Combine all the solutions

x = arcsin (0.68232 \dots) + 2 πn, x = π - arcsin (0.68232 \dots) + 2 πn, x = arcsin (- 0.68232 \dots) + 2 πn, x = π + arcsin (0.68232 \dots) + 2 πn

Verify solutions by plugging them into the original equation

Check the solutions by plugging them into

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

Remove the ones that don't agree with the equation.

Check the solution

arcsin (0.68232 \dots) + 2 πn :

True

arcsin (0.68232 \dots) + 2 πn

Plug in

n = 1

arcsin (0.68232 \dots) + 2 π 1

For

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

plug in

x = arcsin (0.68232 \dots) + 2 π 1

cot (arcsin (0.68232 \dots) + 2 π 1) = sin^{2} (arcsin (0.68232 \dots) + 2 π 1)

Refine

0.68232 \dots = 0.68232 \dots

\Rightarrow True

Check the solution

π - arcsin (0.68232 \dots) + 2 πn :

False

π - arcsin (0.68232 \dots) + 2 πn

Plug in

n = 1

π - arcsin (0.68232 \dots) + 2 π 1

For

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

plug in

x = π - arcsin (0.68232 \dots) + 2 π 1

cot (π - arcsin (0.68232 \dots) + 2 π 1) = sin^{2} (π - arcsin (0.68232 \dots) + 2 π 1)

Refine

- 0.68232 \dots = 0.68232 \dots

\Rightarrow False

Check the solution

arcsin (- 0.68232 \dots) + 2 πn :

False

arcsin (- 0.68232 \dots) + 2 πn

Plug in

n = 1

arcsin (- 0.68232 \dots) + 2 π 1

For

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

plug in

x = arcsin (- 0.68232 \dots) + 2 π 1

cot (arcsin (- 0.68232 \dots) + 2 π 1) = sin^{2} (arcsin (- 0.68232 \dots) + 2 π 1)

Refine

- 0.68232 \dots = 0.68232 \dots

\Rightarrow False

Check the solution

π + arcsin (0.68232 \dots) + 2 πn :

True

π + arcsin (0.68232 \dots) + 2 πn

Plug in

n = 1

π + arcsin (0.68232 \dots) + 2 π 1

For

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

plug in

x = π + arcsin (0.68232 \dots) + 2 π 1

cot (π + arcsin (0.68232 \dots) + 2 π 1) = sin^{2} (π + arcsin (0.68232 \dots) + 2 π 1)

Refine

0.68232 \dots = 0.68232 \dots

\Rightarrow True

x = arcsin (0.68232 \dots) + 2 πn, x = π + arcsin (0.68232 \dots) + 2 πn

Show solutions in decimal form

x = 0.97202 \dots + 2 πn, x = π + 0.97202 \dots + 2 πn

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Frequently Asked Questions (FAQ)

What is the general solution for cot(x)=sin^2(x) ?
The general solution for cot(x)=sin^2(x) is x=0.97202…+2pin,x=pi+0.97202…+2pin