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27cos^3x–8=0 la ecuación

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Solución numérica:

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Solución

Ha introducido [src]
      3           
27*cos (x) - 8 = 0
27cos3(x)8=027 \cos^{3}{\left(x \right)} - 8 = 0
Solución detallada
Tenemos la ecuación
27cos3(x)8=027 \cos^{3}{\left(x \right)} - 8 = 0
cambiamos
27cos3(x)8=027 \cos^{3}{\left(x \right)} - 8 = 0
27cos3(x)8=027 \cos^{3}{\left(x \right)} - 8 = 0
Sustituimos
w=cos(x)w = \cos{\left(x \right)}
Tenemos la ecuación
27w38=027 w^{3} - 8 = 0
Ya que la potencia en la ecuación es igual a = 3 - no contiene número par en el numerador, entonces
la ecuación tendrá una raíz real.
Extraigamos la raíz de potencia 3 de las dos partes de la ecuación:
Obtenemos:
273w33=83\sqrt[3]{27} \sqrt[3]{w^{3}} = \sqrt[3]{8}
o
3w=23 w = 2
Dividamos ambos miembros de la ecuación en 3
w = 2 / (3)

Obtenemos la respuesta: w = 2/3

Las demás 2 raíces son complejas.
hacemos el cambio:
z=wz = w
entonces la ecuación será así:
z3=827z^{3} = \frac{8}{27}
Cualquier número complejo se puede presentar que:
z=reipz = r e^{i p}
sustituimos en la ecuación
r3e3ip=827r^{3} e^{3 i p} = \frac{8}{27}
donde
r=23r = \frac{2}{3}
- módulo del número complejo
Sustituyamos r:
e3ip=1e^{3 i p} = 1
Usando la fórmula de Euler hallemos las raíces para p
isin(3p)+cos(3p)=1i \sin{\left(3 p \right)} + \cos{\left(3 p \right)} = 1
es decir
cos(3p)=1\cos{\left(3 p \right)} = 1
y
sin(3p)=0\sin{\left(3 p \right)} = 0
entonces
p=2πN3p = \frac{2 \pi N}{3}
donde N=0,1,2,3,...
Seleccionando los valores de N y sustituyendo p en la fórmula para z
Es decir, la solución será para z:
z1=23z_{1} = \frac{2}{3}
z2=133i3z_{2} = - \frac{1}{3} - \frac{\sqrt{3} i}{3}
z3=13+3i3z_{3} = - \frac{1}{3} + \frac{\sqrt{3} i}{3}
hacemos cambio inverso
z=wz = w
w=zw = z

Entonces la respuesta definitiva es:
w1=23w_{1} = \frac{2}{3}
w2=133i3w_{2} = - \frac{1}{3} - \frac{\sqrt{3} i}{3}
w3=13+3i3w_{3} = - \frac{1}{3} + \frac{\sqrt{3} i}{3}
hacemos cambio inverso
cos(x)=w\cos{\left(x \right)} = w
Tenemos la ecuación
cos(x)=w\cos{\left(x \right)} = w
es la ecuación trigonométrica más simple
Esta ecuación se reorganiza en
x=πn+acos(w)x = \pi n + \operatorname{acos}{\left(w \right)}
x=πn+acos(w)πx = \pi n + \operatorname{acos}{\left(w \right)} - \pi
O
x=πn+acos(w)x = \pi n + \operatorname{acos}{\left(w \right)}
x=πn+acos(w)πx = \pi n + \operatorname{acos}{\left(w \right)} - \pi
, donde n es cualquier número entero
sustituimos w:
x1=πn+acos(w1)x_{1} = \pi n + \operatorname{acos}{\left(w_{1} \right)}
x1=πn+acos(23)x_{1} = \pi n + \operatorname{acos}{\left(\frac{2}{3} \right)}
x1=πn+acos(23)x_{1} = \pi n + \operatorname{acos}{\left(\frac{2}{3} \right)}
x2=πn+acos(w1)πx_{2} = \pi n + \operatorname{acos}{\left(w_{1} \right)} - \pi
x2=πnπ+acos(23)x_{2} = \pi n - \pi + \operatorname{acos}{\left(\frac{2}{3} \right)}
x2=πnπ+acos(23)x_{2} = \pi n - \pi + \operatorname{acos}{\left(\frac{2}{3} \right)}
Gráfica
0-80-60-40-2020406080-100100-5050
Suma y producto de raíces [src]
suma
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                                    |    |  1   I*\/ 3 ||              |    |  1   I*\/ 3 ||       |    |  1   I*\/ 3 ||              |    |  1   I*\/ 3 ||       |    |  1   I*\/ 3 ||     |    |  1   I*\/ 3 ||       |    |  1   I*\/ 3 ||     |    |  1   I*\/ 3 ||
-acos(2/3) + 2*pi + acos(2/3) + - re|acos|- - - -------|| + 2*pi - I*im|acos|- - - -------|| + - re|acos|- - + -------|| + 2*pi - I*im|acos|- - + -------|| + I*im|acos|- - - -------|| + re|acos|- - - -------|| + I*im|acos|- - + -------|| + re|acos|- - + -------||
                                    \    \  3      3   //              \    \  3      3   //       \    \  3      3   //              \    \  3      3   //       \    \  3      3   //     \    \  3      3   //       \    \  3      3   //     \    \  3      3   //
(re(acos(13+3i3))+iim(acos(13+3i3)))+((((acos(23)+(acos(23)+2π))+(re(acos(133i3))+2πiim(acos(133i3))))+(re(acos(13+3i3))+2πiim(acos(13+3i3))))+(re(acos(133i3))+iim(acos(133i3))))\left(\operatorname{re}{\left(\operatorname{acos}{\left(- \frac{1}{3} + \frac{\sqrt{3} i}{3} \right)}\right)} + i \operatorname{im}{\left(\operatorname{acos}{\left(- \frac{1}{3} + \frac{\sqrt{3} i}{3} \right)}\right)}\right) + \left(\left(\left(\left(\operatorname{acos}{\left(\frac{2}{3} \right)} + \left(- \operatorname{acos}{\left(\frac{2}{3} \right)} + 2 \pi\right)\right) + \left(- \operatorname{re}{\left(\operatorname{acos}{\left(- \frac{1}{3} - \frac{\sqrt{3} i}{3} \right)}\right)} + 2 \pi - i \operatorname{im}{\left(\operatorname{acos}{\left(- \frac{1}{3} - \frac{\sqrt{3} i}{3} \right)}\right)}\right)\right) + \left(- \operatorname{re}{\left(\operatorname{acos}{\left(- \frac{1}{3} + \frac{\sqrt{3} i}{3} \right)}\right)} + 2 \pi - i \operatorname{im}{\left(\operatorname{acos}{\left(- \frac{1}{3} + \frac{\sqrt{3} i}{3} \right)}\right)}\right)\right) + \left(\operatorname{re}{\left(\operatorname{acos}{\left(- \frac{1}{3} - \frac{\sqrt{3} i}{3} \right)}\right)} + i \operatorname{im}{\left(\operatorname{acos}{\left(- \frac{1}{3} - \frac{\sqrt{3} i}{3} \right)}\right)}\right)\right)
=
6*pi
6π6 \pi
producto
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                              |    |    |  1   I*\/ 3 ||              |    |  1   I*\/ 3 ||| |    |    |  1   I*\/ 3 ||              |    |  1   I*\/ 3 ||| |    |    |  1   I*\/ 3 ||     |    |  1   I*\/ 3 ||| |    |    |  1   I*\/ 3 ||     |    |  1   I*\/ 3 |||
(-acos(2/3) + 2*pi)*acos(2/3)*|- re|acos|- - - -------|| + 2*pi - I*im|acos|- - - -------|||*|- re|acos|- - + -------|| + 2*pi - I*im|acos|- - + -------|||*|I*im|acos|- - - -------|| + re|acos|- - - -------|||*|I*im|acos|- - + -------|| + re|acos|- - + -------|||
                              \    \    \  3      3   //              \    \  3      3   /// \    \    \  3      3   //              \    \  3      3   /// \    \    \  3      3   //     \    \  3      3   /// \    \    \  3      3   //     \    \  3      3   ///
(acos(23)+2π)acos(23)(re(acos(133i3))+2πiim(acos(133i3)))(re(acos(13+3i3))+2πiim(acos(13+3i3)))(re(acos(133i3))+iim(acos(133i3)))(re(acos(13+3i3))+iim(acos(13+3i3)))\left(- \operatorname{acos}{\left(\frac{2}{3} \right)} + 2 \pi\right) \operatorname{acos}{\left(\frac{2}{3} \right)} \left(- \operatorname{re}{\left(\operatorname{acos}{\left(- \frac{1}{3} - \frac{\sqrt{3} i}{3} \right)}\right)} + 2 \pi - i \operatorname{im}{\left(\operatorname{acos}{\left(- \frac{1}{3} - \frac{\sqrt{3} i}{3} \right)}\right)}\right) \left(- \operatorname{re}{\left(\operatorname{acos}{\left(- \frac{1}{3} + \frac{\sqrt{3} i}{3} \right)}\right)} + 2 \pi - i \operatorname{im}{\left(\operatorname{acos}{\left(- \frac{1}{3} + \frac{\sqrt{3} i}{3} \right)}\right)}\right) \left(\operatorname{re}{\left(\operatorname{acos}{\left(- \frac{1}{3} - \frac{\sqrt{3} i}{3} \right)}\right)} + i \operatorname{im}{\left(\operatorname{acos}{\left(- \frac{1}{3} - \frac{\sqrt{3} i}{3} \right)}\right)}\right) \left(\operatorname{re}{\left(\operatorname{acos}{\left(- \frac{1}{3} + \frac{\sqrt{3} i}{3} \right)}\right)} + i \operatorname{im}{\left(\operatorname{acos}{\left(- \frac{1}{3} + \frac{\sqrt{3} i}{3} \right)}\right)}\right)
=
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                    |    |    |  1   I*\/ 3 ||     |    |  1   I*\/ 3 ||| |    |    |  1   I*\/ 3 ||     |    |  1   I*\/ 3 ||| |            |    |  1   I*\/ 3 ||     |    |  1   I*\/ 3 ||| |            |    |  1   I*\/ 3 ||     |    |  1   I*\/ 3 |||          
(-acos(2/3) + 2*pi)*|I*im|acos|- - - -------|| + re|acos|- - - -------|||*|I*im|acos|- - + -------|| + re|acos|- - + -------|||*|-2*pi + I*im|acos|- - - -------|| + re|acos|- - - -------|||*|-2*pi + I*im|acos|- - + -------|| + re|acos|- - + -------|||*acos(2/3)
                    \    \    \  3      3   //     \    \  3      3   /// \    \    \  3      3   //     \    \  3      3   /// \            \    \  3      3   //     \    \  3      3   /// \            \    \  3      3   //     \    \  3      3   ///          
(re(acos(133i3))+iim(acos(133i3)))(re(acos(13+3i3))+iim(acos(13+3i3)))(acos(23)+2π)(2π+re(acos(133i3))+iim(acos(133i3)))(2π+re(acos(13+3i3))+iim(acos(13+3i3)))acos(23)\left(\operatorname{re}{\left(\operatorname{acos}{\left(- \frac{1}{3} - \frac{\sqrt{3} i}{3} \right)}\right)} + i \operatorname{im}{\left(\operatorname{acos}{\left(- \frac{1}{3} - \frac{\sqrt{3} i}{3} \right)}\right)}\right) \left(\operatorname{re}{\left(\operatorname{acos}{\left(- \frac{1}{3} + \frac{\sqrt{3} i}{3} \right)}\right)} + i \operatorname{im}{\left(\operatorname{acos}{\left(- \frac{1}{3} + \frac{\sqrt{3} i}{3} \right)}\right)}\right) \left(- \operatorname{acos}{\left(\frac{2}{3} \right)} + 2 \pi\right) \left(- 2 \pi + \operatorname{re}{\left(\operatorname{acos}{\left(- \frac{1}{3} - \frac{\sqrt{3} i}{3} \right)}\right)} + i \operatorname{im}{\left(\operatorname{acos}{\left(- \frac{1}{3} - \frac{\sqrt{3} i}{3} \right)}\right)}\right) \left(- 2 \pi + \operatorname{re}{\left(\operatorname{acos}{\left(- \frac{1}{3} + \frac{\sqrt{3} i}{3} \right)}\right)} + i \operatorname{im}{\left(\operatorname{acos}{\left(- \frac{1}{3} + \frac{\sqrt{3} i}{3} \right)}\right)}\right) \operatorname{acos}{\left(\frac{2}{3} \right)}
(-acos(2/3) + 2*pi)*(i*im(acos(-1/3 - i*sqrt(3)/3)) + re(acos(-1/3 - i*sqrt(3)/3)))*(i*im(acos(-1/3 + i*sqrt(3)/3)) + re(acos(-1/3 + i*sqrt(3)/3)))*(-2*pi + i*im(acos(-1/3 - i*sqrt(3)/3)) + re(acos(-1/3 - i*sqrt(3)/3)))*(-2*pi + i*im(acos(-1/3 + i*sqrt(3)/3)) + re(acos(-1/3 + i*sqrt(3)/3)))*acos(2/3)
Respuesta rápida [src]
x1 = -acos(2/3) + 2*pi
x1=acos(23)+2πx_{1} = - \operatorname{acos}{\left(\frac{2}{3} \right)} + 2 \pi
x2 = acos(2/3)
x2=acos(23)x_{2} = \operatorname{acos}{\left(\frac{2}{3} \right)}
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         |    |  1   I*\/ 3 ||              |    |  1   I*\/ 3 ||
x3 = - re|acos|- - - -------|| + 2*pi - I*im|acos|- - - -------||
         \    \  3      3   //              \    \  3      3   //
x3=re(acos(133i3))+2πiim(acos(133i3))x_{3} = - \operatorname{re}{\left(\operatorname{acos}{\left(- \frac{1}{3} - \frac{\sqrt{3} i}{3} \right)}\right)} + 2 \pi - i \operatorname{im}{\left(\operatorname{acos}{\left(- \frac{1}{3} - \frac{\sqrt{3} i}{3} \right)}\right)}
         /    /          ___\\              /    /          ___\\
         |    |  1   I*\/ 3 ||              |    |  1   I*\/ 3 ||
x4 = - re|acos|- - + -------|| + 2*pi - I*im|acos|- - + -------||
         \    \  3      3   //              \    \  3      3   //
x4=re(acos(13+3i3))+2πiim(acos(13+3i3))x_{4} = - \operatorname{re}{\left(\operatorname{acos}{\left(- \frac{1}{3} + \frac{\sqrt{3} i}{3} \right)}\right)} + 2 \pi - i \operatorname{im}{\left(\operatorname{acos}{\left(- \frac{1}{3} + \frac{\sqrt{3} i}{3} \right)}\right)}
         /    /          ___\\     /    /          ___\\
         |    |  1   I*\/ 3 ||     |    |  1   I*\/ 3 ||
x5 = I*im|acos|- - - -------|| + re|acos|- - - -------||
         \    \  3      3   //     \    \  3      3   //
x5=re(acos(133i3))+iim(acos(133i3))x_{5} = \operatorname{re}{\left(\operatorname{acos}{\left(- \frac{1}{3} - \frac{\sqrt{3} i}{3} \right)}\right)} + i \operatorname{im}{\left(\operatorname{acos}{\left(- \frac{1}{3} - \frac{\sqrt{3} i}{3} \right)}\right)}
         /    /          ___\\     /    /          ___\\
         |    |  1   I*\/ 3 ||     |    |  1   I*\/ 3 ||
x6 = I*im|acos|- - + -------|| + re|acos|- - + -------||
         \    \  3      3   //     \    \  3      3   //
x6=re(acos(13+3i3))+iim(acos(13+3i3))x_{6} = \operatorname{re}{\left(\operatorname{acos}{\left(- \frac{1}{3} + \frac{\sqrt{3} i}{3} \right)}\right)} + i \operatorname{im}{\left(\operatorname{acos}{\left(- \frac{1}{3} + \frac{\sqrt{3} i}{3} \right)}\right)}
x6 = re(acos(-1/3 + sqrt(3)*i/3)) + i*im(acos(-1/3 + sqrt(3)*i/3))
Respuesta numérica [src]
x1 = -99.6898962443055
x2 = 69.9561070495434
x3 = 61.9907844012279
x4 = 214.469369114674
x5 = 49.4244137868688
x6 = 82.5224776639025
x7 = -61.9907844012279
x8 = 18.0084872509708
x9 = -49.4244137868688
x10 = -80.8403403227667
x11 = -69.9561070495434
x12 = 99.6898962443055
x13 = -55.7075990940483
x14 = 87.1235256299463
x15 = 101.372033585441
x16 = -18.0084872509708
x17 = 74.5571550155871
x18 = 76.239292356723
x19 = -13.4074392849271
x20 = 63.6729217423638
x21 = 25.9738098992863
x22 = -57.3897364351842
x23 = 19.6906245921067
x24 = 24.2916725581504
x25 = 95.0888482782617
x26 = -24.2916725581504
x27 = 124.822637473024
x28 = -19.6906245921067
x29 = 44.823365820825
x30 = -87.1235256299463
x31 = 55.7075990940483
x32 = -82.5224776639025
x33 = 68.2739697084075
x34 = -44.823365820825
x35 = 51.1065511280046
x36 = -36.8580431725096
x37 = -76.239292356723
x38 = -63.6729217423638
x39 = -25.9738098992863
x40 = 38.5401805136454
x41 = 0.84106867056793
x42 = -43.1412284796892
x43 = 88.8056629710821
x44 = 32.2569952064659
x45 = -5.44211663661166
x46 = -93.4067109371259
x47 = 5.44211663661166
x48 = -68.2739697084075
x49 = 30.57485786533
x50 = 7.12425397774752
x51 = 11.7253019437912
x52 = 105.973081551485
x53 = -38.5401805136454
x54 = -11.7253019437912
x55 = -32.2569952064659
x56 = 43.1412284796892
x57 = -101.372033585441
x58 = -88.8056629710821
x59 = -0.84106867056793
x59 = -0.84106867056793