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4*cos^4x+9*cos^2x-1=0 equation

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Numerical solution:

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The solution

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     4           2           
4*cos (x) + 9*cos (x) - 1 = 0
$$\left(4 \cos^{4}{\left(x \right)} + 9 \cos^{2}{\left(x \right)}\right) - 1 = 0$$
Detail solution
Given the equation
$$\left(4 \cos^{4}{\left(x \right)} + 9 \cos^{2}{\left(x \right)}\right) - 1 = 0$$
transform
$$4 \cos^{4}{\left(x \right)} + 9 \cos^{2}{\left(x \right)} - 1 = 0$$
$$\left(4 \cos^{4}{\left(x \right)} + 9 \cos^{2}{\left(x \right)}\right) - 1 = 0$$
Do replacement
$$w = \cos{\left(x \right)}$$
Given the equation:
$$4 w^{4} + 9 w^{2} - 1 = 0$$
Do replacement
$$v = w^{2}$$
then the equation will be the:
$$4 v^{2} + 9 v - 1 = 0$$
This equation is of the form
a*v^2 + b*v + c = 0

A quadratic equation can be solved
using the discriminant.
The roots of the quadratic equation:
$$v_{1} = \frac{\sqrt{D} - b}{2 a}$$
$$v_{2} = \frac{- \sqrt{D} - b}{2 a}$$
where D = b^2 - 4*a*c - it is the discriminant.
Because
$$a = 4$$
$$b = 9$$
$$c = -1$$
, then
D = b^2 - 4 * a * c = 

(9)^2 - 4 * (4) * (-1) = 97

Because D > 0, then the equation has two roots.
v1 = (-b + sqrt(D)) / (2*a)

v2 = (-b - sqrt(D)) / (2*a)

or
$$v_{1} = - \frac{9}{8} + \frac{\sqrt{97}}{8}$$
$$v_{2} = - \frac{\sqrt{97}}{8} - \frac{9}{8}$$
The final answer:
Because
$$v = w^{2}$$
then
$$w_{1} = \sqrt{v_{1}}$$
$$w_{2} = - \sqrt{v_{1}}$$
$$w_{3} = \sqrt{v_{2}}$$
$$w_{4} = - \sqrt{v_{2}}$$
then:
$$w_{1} = $$
$$\frac{0}{1} + \frac{\left(- \frac{9}{8} + \frac{\sqrt{97}}{8}\right)^{\frac{1}{2}}}{1} = \sqrt{- \frac{9}{8} + \frac{\sqrt{97}}{8}}$$
$$w_{2} = $$
$$\frac{\left(-1\right) \left(- \frac{9}{8} + \frac{\sqrt{97}}{8}\right)^{\frac{1}{2}}}{1} + \frac{0}{1} = - \sqrt{- \frac{9}{8} + \frac{\sqrt{97}}{8}}$$
$$w_{3} = $$
$$\frac{0}{1} + \frac{\left(- \frac{\sqrt{97}}{8} - \frac{9}{8}\right)^{\frac{1}{2}}}{1} = \sqrt{- \frac{\sqrt{97}}{8} - \frac{9}{8}}$$
$$w_{4} = $$
$$\frac{0}{1} + \frac{\left(-1\right) \left(- \frac{\sqrt{97}}{8} - \frac{9}{8}\right)^{\frac{1}{2}}}{1} = - \sqrt{- \frac{\sqrt{97}}{8} - \frac{9}{8}}$$
do backward replacement
$$\cos{\left(x \right)} = w$$
Given the equation
$$\cos{\left(x \right)} = w$$
- this is the simplest trigonometric equation
This equation is transformed to
$$x = \pi n + \operatorname{acos}{\left(w \right)}$$
$$x = \pi n + \operatorname{acos}{\left(w \right)} - \pi$$
Or
$$x = \pi n + \operatorname{acos}{\left(w \right)}$$
$$x = \pi n + \operatorname{acos}{\left(w \right)} - \pi$$
, where n - is a integer
substitute w:
$$x_{1} = \pi n + \operatorname{acos}{\left(w_{1} \right)}$$
$$x_{1} = \pi n + \operatorname{acos}{\left(- \frac{9}{8} + \frac{\sqrt{97}}{8} \right)}$$
$$x_{1} = \pi n + \operatorname{acos}{\left(- \frac{9}{8} + \frac{\sqrt{97}}{8} \right)}$$
$$x_{2} = \pi n + \operatorname{acos}{\left(w_{2} \right)}$$
$$x_{2} = \pi n + \operatorname{acos}{\left(- \frac{\sqrt{97}}{8} - \frac{9}{8} \right)}$$
$$x_{2} = \pi n + \operatorname{acos}{\left(- \frac{\sqrt{97}}{8} - \frac{9}{8} \right)}$$
$$x_{3} = \pi n + \operatorname{acos}{\left(w_{1} \right)} - \pi$$
$$x_{3} = \pi n - \pi + \operatorname{acos}{\left(- \frac{9}{8} + \frac{\sqrt{97}}{8} \right)}$$
$$x_{3} = \pi n - \pi + \operatorname{acos}{\left(- \frac{9}{8} + \frac{\sqrt{97}}{8} \right)}$$
$$x_{4} = \pi n + \operatorname{acos}{\left(w_{2} \right)} - \pi$$
$$x_{4} = \pi n - \pi + \operatorname{acos}{\left(- \frac{\sqrt{97}}{8} - \frac{9}{8} \right)}$$
$$x_{4} = \pi n - \pi + \operatorname{acos}{\left(- \frac{\sqrt{97}}{8} - \frac{9}{8} \right)}$$
The graph
Rapid solution [src]
           /      ______________\       
           |     /         ____ |       
           |    /    9   \/ 97  |       
x1 = - acos|-  /   - - + ------ | + 2*pi
           \ \/      8     8    /       
$$x_{1} = - \operatorname{acos}{\left(- \sqrt{- \frac{9}{8} + \frac{\sqrt{97}}{8}} \right)} + 2 \pi$$
           /     ______________\       
           |    /         ____ |       
           |   /    9   \/ 97  |       
x2 = - acos|  /   - - + ------ | + 2*pi
           \\/      8     8    /       
$$x_{2} = - \operatorname{acos}{\left(\sqrt{- \frac{9}{8} + \frac{\sqrt{97}}{8}} \right)} + 2 \pi$$
         /      ______________\
         |     /         ____ |
         |    /    9   \/ 97  |
x3 = acos|-  /   - - + ------ |
         \ \/      8     8    /
$$x_{3} = \operatorname{acos}{\left(- \sqrt{- \frac{9}{8} + \frac{\sqrt{97}}{8}} \right)}$$
         /     ______________\
         |    /         ____ |
         |   /    9   \/ 97  |
x4 = acos|  /   - - + ------ |
         \\/      8     8    /
$$x_{4} = \operatorname{acos}{\left(\sqrt{- \frac{9}{8} + \frac{\sqrt{97}}{8}} \right)}$$
                 /     ____________\
                 |    /       ____ |
     pi          |   /  9   \/ 97  |
x5 = -- - I*asinh|  /   - + ------ |
     2           \\/    8     8    /
$$x_{5} = \frac{\pi}{2} - i \operatorname{asinh}{\left(\sqrt{\frac{9}{8} + \frac{\sqrt{97}}{8}} \right)}$$
                 /     ____________\
                 |    /       ____ |
     pi          |   /  9   \/ 97  |
x6 = -- + I*asinh|  /   - + ------ |
     2           \\/    8     8    /
$$x_{6} = \frac{\pi}{2} + i \operatorname{asinh}{\left(\sqrt{\frac{9}{8} + \frac{\sqrt{97}}{8}} \right)}$$
                   /     ____________\
                   |    /       ____ |
     3*pi          |   /  9   \/ 97  |
x7 = ---- - I*asinh|  /   - + ------ |
      2            \\/    8     8    /
$$x_{7} = \frac{3 \pi}{2} - i \operatorname{asinh}{\left(\sqrt{\frac{9}{8} + \frac{\sqrt{97}}{8}} \right)}$$
                   /     ____________\
                   |    /       ____ |
     3*pi          |   /  9   \/ 97  |
x8 = ---- + I*asinh|  /   - + ------ |
      2            \\/    8     8    /
$$x_{8} = \frac{3 \pi}{2} + i \operatorname{asinh}{\left(\sqrt{\frac{9}{8} + \frac{\sqrt{97}}{8}} \right)}$$
x8 = 3*pi/2 + i*asinh(sqrt(9/8 + sqrt(97)/8))
Sum and product of roots [src]
sum
      /      ______________\                /     ______________\              /      ______________\       /     ______________\               /     ____________\               /     ____________\                 /     ____________\                 /     ____________\
      |     /         ____ |                |    /         ____ |              |     /         ____ |       |    /         ____ |               |    /       ____ |               |    /       ____ |                 |    /       ____ |                 |    /       ____ |
      |    /    9   \/ 97  |                |   /    9   \/ 97  |              |    /    9   \/ 97  |       |   /    9   \/ 97  |   pi          |   /  9   \/ 97  |   pi          |   /  9   \/ 97  |   3*pi          |   /  9   \/ 97  |   3*pi          |   /  9   \/ 97  |
- acos|-  /   - - + ------ | + 2*pi + - acos|  /   - - + ------ | + 2*pi + acos|-  /   - - + ------ | + acos|  /   - - + ------ | + -- - I*asinh|  /   - + ------ | + -- + I*asinh|  /   - + ------ | + ---- - I*asinh|  /   - + ------ | + ---- + I*asinh|  /   - + ------ |
      \ \/      8     8    /                \\/      8     8    /              \ \/      8     8    /       \\/      8     8    /   2           \\/    8     8    /   2           \\/    8     8    /    2            \\/    8     8    /    2            \\/    8     8    /
$$\left(\left(\frac{3 \pi}{2} - i \operatorname{asinh}{\left(\sqrt{\frac{9}{8} + \frac{\sqrt{97}}{8}} \right)}\right) + \left(\left(\left(\operatorname{acos}{\left(\sqrt{- \frac{9}{8} + \frac{\sqrt{97}}{8}} \right)} + \left(\operatorname{acos}{\left(- \sqrt{- \frac{9}{8} + \frac{\sqrt{97}}{8}} \right)} + \left(\left(- \operatorname{acos}{\left(- \sqrt{- \frac{9}{8} + \frac{\sqrt{97}}{8}} \right)} + 2 \pi\right) + \left(- \operatorname{acos}{\left(\sqrt{- \frac{9}{8} + \frac{\sqrt{97}}{8}} \right)} + 2 \pi\right)\right)\right)\right) + \left(\frac{\pi}{2} - i \operatorname{asinh}{\left(\sqrt{\frac{9}{8} + \frac{\sqrt{97}}{8}} \right)}\right)\right) + \left(\frac{\pi}{2} + i \operatorname{asinh}{\left(\sqrt{\frac{9}{8} + \frac{\sqrt{97}}{8}} \right)}\right)\right)\right) + \left(\frac{3 \pi}{2} + i \operatorname{asinh}{\left(\sqrt{\frac{9}{8} + \frac{\sqrt{97}}{8}} \right)}\right)$$
=
8*pi
$$8 \pi$$
product
/      /      ______________\       \ /      /     ______________\       \     /      ______________\     /     ______________\ /            /     ____________\\ /            /     ____________\\ /              /     ____________\\ /              /     ____________\\
|      |     /         ____ |       | |      |    /         ____ |       |     |     /         ____ |     |    /         ____ | |            |    /       ____ || |            |    /       ____ || |              |    /       ____ || |              |    /       ____ ||
|      |    /    9   \/ 97  |       | |      |   /    9   \/ 97  |       |     |    /    9   \/ 97  |     |   /    9   \/ 97  | |pi          |   /  9   \/ 97  || |pi          |   /  9   \/ 97  || |3*pi          |   /  9   \/ 97  || |3*pi          |   /  9   \/ 97  ||
|- acos|-  /   - - + ------ | + 2*pi|*|- acos|  /   - - + ------ | + 2*pi|*acos|-  /   - - + ------ |*acos|  /   - - + ------ |*|-- - I*asinh|  /   - + ------ ||*|-- + I*asinh|  /   - + ------ ||*|---- - I*asinh|  /   - + ------ ||*|---- + I*asinh|  /   - + ------ ||
\      \ \/      8     8    /       / \      \\/      8     8    /       /     \ \/      8     8    /     \\/      8     8    / \2           \\/    8     8    // \2           \\/    8     8    // \ 2            \\/    8     8    // \ 2            \\/    8     8    //
$$\left(- \operatorname{acos}{\left(- \sqrt{- \frac{9}{8} + \frac{\sqrt{97}}{8}} \right)} + 2 \pi\right) \left(- \operatorname{acos}{\left(\sqrt{- \frac{9}{8} + \frac{\sqrt{97}}{8}} \right)} + 2 \pi\right) \operatorname{acos}{\left(- \sqrt{- \frac{9}{8} + \frac{\sqrt{97}}{8}} \right)} \operatorname{acos}{\left(\sqrt{- \frac{9}{8} + \frac{\sqrt{97}}{8}} \right)} \left(\frac{\pi}{2} - i \operatorname{asinh}{\left(\sqrt{\frac{9}{8} + \frac{\sqrt{97}}{8}} \right)}\right) \left(\frac{\pi}{2} + i \operatorname{asinh}{\left(\sqrt{\frac{9}{8} + \frac{\sqrt{97}}{8}} \right)}\right) \left(\frac{3 \pi}{2} - i \operatorname{asinh}{\left(\sqrt{\frac{9}{8} + \frac{\sqrt{97}}{8}} \right)}\right) \left(\frac{3 \pi}{2} + i \operatorname{asinh}{\left(\sqrt{\frac{9}{8} + \frac{\sqrt{97}}{8}} \right)}\right)$$
=
/              /         ____________\\ /              /         ____________\\ /      /          _____________ \       \ /      /         _____________\       \ /                /         ____________\\ /                /         ____________\\     /          _____________ \     /         _____________\
|              |  ___   /       ____ || |              |  ___   /       ____ || |      |   ___   /        ____  |       | |      |  ___   /        ____ |       | |                |  ___   /       ____ || |                |  ___   /       ____ ||     |   ___   /        ____  |     |  ___   /        ____ |
|              |\/ 2 *\/  9 + \/ 97  || |              |\/ 2 *\/  9 + \/ 97  || |      |-\/ 2 *\/  -9 + \/ 97   |       | |      |\/ 2 *\/  -9 + \/ 97  |       | |                |\/ 2 *\/  9 + \/ 97  || |                |\/ 2 *\/  9 + \/ 97  ||     |-\/ 2 *\/  -9 + \/ 97   |     |\/ 2 *\/  -9 + \/ 97  |
|pi - 2*I*asinh|---------------------||*|pi + 2*I*asinh|---------------------||*|- acos|------------------------| + 2*pi|*|- acos|----------------------| + 2*pi|*|3*pi - 2*I*asinh|---------------------||*|3*pi + 2*I*asinh|---------------------||*acos|------------------------|*acos|----------------------|
\              \          4          // \              \          4          // \      \           4            /       / \      \          4           /       / \                \          4          // \                \          4          //     \           4            /     \          4           /
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                                                                                        16                                                                                                                                                       
$$\frac{\left(\pi - 2 i \operatorname{asinh}{\left(\frac{\sqrt{2} \sqrt{9 + \sqrt{97}}}{4} \right)}\right) \left(\pi + 2 i \operatorname{asinh}{\left(\frac{\sqrt{2} \sqrt{9 + \sqrt{97}}}{4} \right)}\right) \left(3 \pi - 2 i \operatorname{asinh}{\left(\frac{\sqrt{2} \sqrt{9 + \sqrt{97}}}{4} \right)}\right) \left(3 \pi + 2 i \operatorname{asinh}{\left(\frac{\sqrt{2} \sqrt{9 + \sqrt{97}}}{4} \right)}\right) \left(- \operatorname{acos}{\left(- \frac{\sqrt{2} \sqrt{-9 + \sqrt{97}}}{4} \right)} + 2 \pi\right) \left(- \operatorname{acos}{\left(\frac{\sqrt{2} \sqrt{-9 + \sqrt{97}}}{4} \right)} + 2 \pi\right) \operatorname{acos}{\left(- \frac{\sqrt{2} \sqrt{-9 + \sqrt{97}}}{4} \right)} \operatorname{acos}{\left(\frac{\sqrt{2} \sqrt{-9 + \sqrt{97}}}{4} \right)}}{16}$$
(pi - 2*i*asinh(sqrt(2)*sqrt(9 + sqrt(97))/4))*(pi + 2*i*asinh(sqrt(2)*sqrt(9 + sqrt(97))/4))*(-acos(-sqrt(2)*sqrt(-9 + sqrt(97))/4) + 2*pi)*(-acos(sqrt(2)*sqrt(-9 + sqrt(97))/4) + 2*pi)*(3*pi - 2*i*asinh(sqrt(2)*sqrt(9 + sqrt(97))/4))*(3*pi + 2*i*asinh(sqrt(2)*sqrt(9 + sqrt(97))/4))*acos(-sqrt(2)*sqrt(-9 + sqrt(97))/4)*acos(sqrt(2)*sqrt(-9 + sqrt(97))/4)/16
Numerical answer [src]
x1 = -1.90259173789352
x2 = 11.3273696986629
x3 = -42.0797054123636
x4 = -4.38059356928607
x5 = 76.6372246018513
x6 = -70.3540392946717
x7 = 491.327454875704
x8 = 4.38059356928607
x9 = 67.8760374632792
x10 = -33.3185182737914
x11 = -93.0087786919975
x12 = -39.601703580971
x13 = 38.9381127587738
x14 = 51.504483373133
x15 = -52.1680741953302
x16 = 8.1857770450731
x17 = 74.1592227704588
x18 = 23.8937403130221
x19 = 70.3540392946717
x20 = 42.7432962345608
x21 = 89.8671860384077
x22 = 26.3717421444146
x23 = 108.716741959946
x24 = 1.90259173789352
x25 = 54.6460760267228
x26 = 23.2301494908248
x27 = 16.9469641836452
x28 = 92.3451878698003
x29 = -17.6105550058425
x30 = -49.0264815417404
x31 = 98.6283731769799
x32 = -10.6637788764657
x33 = 45.8848888881506
x34 = -92.3451878698003
x35 = 39.601703580971
x36 = -8.1857770450731
x37 = 20.088556837235
x38 = 55.30966684892
x39 = 17.6105550058425
x40 = 77.3008154240486
x41 = 64.0708539874921
x42 = 60.9292613339023
x43 = 30.1769256202017
x44 = 32.6549274515942
x45 = 86.0620025626207
x46 = -89.8671860384077
x47 = -27.0353329666119
x48 = -5.04418439148331
x49 = -57.7876686803126
x50 = -23.8937403130221
x51 = -76.6372246018513
x52 = -79.7788172554411
x53 = -121.283112574306
x54 = 42.0797054123636
x55 = 96.1503713455873
x56 = -11.3273696986629
x57 = 48.3628907195432
x58 = 82.9204099090309
x59 = -83.5840007312281
x60 = 99.2919639991771
x61 = -98.6283731769799
x62 = -64.0708539874921
x63 = -20.088556837235
x64 = -48.3628907195432
x65 = -67.8760374632792
x66 = -96.1503713455873
x67 = 83.5840007312281
x68 = 71.017630116869
x69 = -71.017630116869
x70 = -35.796520105184
x71 = -26.3717421444146
x72 = -32.6549274515942
x73 = -74.1592227704588
x74 = -45.8848888881506
x75 = -77.3008154240486
x76 = -1280.53080174894
x77 = 52.1680741953302
x78 = 33.3185182737914
x79 = -55.30966684892
x80 = 61.5928521560996
x81 = -99.2919639991771
x82 = -13.8053715300554
x83 = 10.6637788764657
x84 = -86.0620025626207
x85 = -61.5928521560996
x86 = -30.1769256202017
x87 = -54.6460760267228
x87 = -54.6460760267228