x^8+1=0 (уравнение)

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    Найду корень уравнения: x^8+1=0

    Решение

    Вы ввели [src]
     8        
    x  + 1 = 0
    x8+1=0x^{8} + 1 = 0
    Подробное решение
    Дано уравнение
    x8+1=0x^{8} + 1 = 0
    Т.к. степень в ур-нии равна = 8 и свободный член = -1 < 0,
    зн. действительных решений у соотв. ур-ния не существует

    Остальные 8 корня(ей) являются комплексными.
    сделаем замену:
    z=xz = x
    тогда ур-ние будет таким:
    z8=1z^{8} = -1
    Любое комплексное число можно представить так:
    z=reipz = r e^{i p}
    подставляем в уравнение
    r8e8ip=1r^{8} e^{8 i p} = -1
    где
    r=1r = 1
    - модуль комплексного числа
    Подставляем r:
    e8ip=1e^{8 i p} = -1
    Используя формулу Эйлера, найдём корни для p
    isin(8p)+cos(8p)=1i \sin{\left(8 p \right)} + \cos{\left(8 p \right)} = -1
    значит
    cos(8p)=1\cos{\left(8 p \right)} = -1
    и
    sin(8p)=0\sin{\left(8 p \right)} = 0
    тогда
    p=πN4+π8p = \frac{\pi N}{4} + \frac{\pi}{8}
    где N=0,1,2,3,...
    Перебирая значения N и подставив p в формулу для z
    Значит, решением будет для z:
    z1=1224+i24+12z_{1} = - \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}} + i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}
    z2=1224i24+12z_{2} = \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}} - i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}
    z3=24+12i1224z_{3} = - \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}} - i \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}
    z4=24+12+i1224z_{4} = \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}} + i \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}
    z5=212242+224+122+2i12242+2i24+122z_{5} = - \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} + \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} i \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} + \frac{\sqrt{2} i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2}
    z6=212242+224+1222i24+122+2i12242z_{6} = \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} + \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} - \frac{\sqrt{2} i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} i \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2}
    z7=224+1222122422i12242+2i24+122z_{7} = - \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} - \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} - \frac{\sqrt{2} i \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} + \frac{\sqrt{2} i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2}
    z8=224+122+2122422i24+1222i12242z_{8} = - \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} - \frac{\sqrt{2} i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} - \frac{\sqrt{2} i \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2}
    делаем обратную замену
    z=xz = x
    x=zx = z

    Тогда, окончательный ответ:
    x1=1224+i24+12x_{1} = - \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}} + i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}
    x2=1224i24+12x_{2} = \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}} - i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}
    x3=24+12i1224x_{3} = - \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}} - i \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}
    x4=24+12+i1224x_{4} = \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}} + i \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}
    x5=212242+224+122+2i12242+2i24+122x_{5} = - \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} + \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} i \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} + \frac{\sqrt{2} i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2}
    x6=212242+224+1222i24+122+2i12242x_{6} = \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} + \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} - \frac{\sqrt{2} i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} i \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2}
    x7=224+1222122422i12242+2i24+122x_{7} = - \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} - \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} - \frac{\sqrt{2} i \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} + \frac{\sqrt{2} i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2}
    x8=224+122+2122422i24+1222i12242x_{8} = - \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} - \frac{\sqrt{2} i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} - \frac{\sqrt{2} i \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2}
    График
    -1.5-1.0-0.50.00.51.01.52.0020
    Быстрый ответ [src]
                ___________          ___________
               /       ___          /       ___ 
              /  1   \/ 2          /  1   \/ 2  
    x1 = -   /   - - -----  + I*  /   - + ----- 
           \/    2     4        \/    2     4   
    x1=1224+i24+12x_{1} = - \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}} + i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}
              ___________          ___________
             /       ___          /       ___ 
            /  1   \/ 2          /  1   \/ 2  
    x2 =   /   - - -----  - I*  /   - + ----- 
         \/    2     4        \/    2     4   
    x2=1224i24+12x_{2} = \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}} - i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}
                ___________          ___________
               /       ___          /       ___ 
              /  1   \/ 2          /  1   \/ 2  
    x3 = -   /   - + -----  - I*  /   - - ----- 
           \/    2     4        \/    2     4   
    x3=24+12i1224x_{3} = - \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}} - i \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}
              ___________          ___________
             /       ___          /       ___ 
            /  1   \/ 2          /  1   \/ 2  
    x4 =   /   - + -----  + I*  /   - - ----- 
         \/    2     4        \/    2     4   
    x4=24+12+i1224x_{4} = \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}} + i \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}
           /           ___________              ___________\              ___________              ___________
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           |  ___    /  1   \/ 2       ___    /  1   \/ 2  |     ___    /  1   \/ 2       ___    /  1   \/ 2  
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           |      \/    2     4            \/    2     4   |         \/    2     4            \/    2     4   
    x5 = I*|---------------------- + ----------------------| + ---------------------- - ----------------------
           \          2                        2           /             2                        2           
    x5=212242+224+122+i(212242+224+122)x_{5} = - \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} + \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + i \left(\frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} + \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2}\right)
           /           ___________              ___________\              ___________              ___________
           |          /       ___              /       ___ |             /       ___              /       ___ 
           |  ___    /  1   \/ 2       ___    /  1   \/ 2  |     ___    /  1   \/ 2       ___    /  1   \/ 2  
           |\/ 2 *  /   - - -----    \/ 2 *  /   - + ----- |   \/ 2 *  /   - - -----    \/ 2 *  /   - + ----- 
           |      \/    2     4            \/    2     4   |         \/    2     4            \/    2     4   
    x6 = I*|---------------------- - ----------------------| + ---------------------- + ----------------------
           \          2                        2           /             2                        2           
    x6=212242+224+122+i(224+122+212242)x_{6} = \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} + \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + i \left(- \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2}\right)
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           |  ___    /  1   \/ 2       ___    /  1   \/ 2  |     ___    /  1   \/ 2       ___    /  1   \/ 2  
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           |      \/    2     4            \/    2     4   |         \/    2     4            \/    2     4   
    x7 = I*|---------------------- - ----------------------| - ---------------------- - ----------------------
           \          2                        2           /             2                        2           
    x7=224+122212242+i(212242+224+122)x_{7} = - \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} - \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} + i \left(- \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} + \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2}\right)
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           |    ___    /  1   \/ 2       ___    /  1   \/ 2  |     ___    /  1   \/ 2       ___    /  1   \/ 2  
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           |        \/    2     4            \/    2     4   |         \/    2     4            \/    2     4   
    x8 = I*|- ---------------------- - ----------------------| + ---------------------- - ----------------------
           \            2                        2           /             2                        2           
    x8=224+122+212242+i(224+122212242)x_{8} = - \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} + i \left(- \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} - \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2}\right)
    Сумма и произведение корней [src]
    сумма
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    0 + -   /   - - -----  + I*  /   - + -----  +   /   - - -----  - I*  /   - + -----  + -   /   - + -----  - I*  /   - - -----  +   /   - + -----  + I*  /   - - -----  + I*|---------------------- + ----------------------| + ---------------------- - ---------------------- + I*|---------------------- - ----------------------| + ---------------------- + ---------------------- + I*|---------------------- - ----------------------| - ---------------------- - ---------------------- + I*|- ---------------------- - ----------------------| + ---------------------- - ----------------------
          \/    2     4        \/    2     4      \/    2     4        \/    2     4        \/    2     4        \/    2     4      \/    2     4        \/    2     4        \          2                        2           /             2                        2                \          2                        2           /             2                        2                \          2                        2           /             2                        2                \            2                        2           /             2                        2           
    (224+122+212242+i(224+122212242))(224+122+212242i(212242+224+122)i(212242+224+122)i(224+122+212242))\left(- \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} + i \left(- \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} - \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2}\right)\right) - \left(- \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} - i \left(\frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} + \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2}\right) - i \left(- \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} + \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2}\right) - i \left(- \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2}\right)\right)
    =
      /           ___________              ___________\     /           ___________              ___________\     /           ___________              ___________\     /             ___________              ___________\
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      |\/ 2 *  /   - - -----    \/ 2 *  /   - + ----- |     |\/ 2 *  /   - - -----    \/ 2 *  /   - + ----- |     |\/ 2 *  /   - + -----    \/ 2 *  /   - - ----- |     |  \/ 2 *  /   - - -----    \/ 2 *  /   - + ----- |
      |      \/    2     4            \/    2     4   |     |      \/    2     4            \/    2     4   |     |      \/    2     4            \/    2     4   |     |        \/    2     4            \/    2     4   |
    I*|---------------------- + ----------------------| + I*|---------------------- - ----------------------| + I*|---------------------- - ----------------------| + I*|- ---------------------- - ----------------------|
      \          2                        2           /     \          2                        2           /     \          2                        2           /     \            2                        2           /
    i(224+122212242)+i(224+122+212242)+i(212242+224+122)+i(212242+224+122)i \left(- \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} - \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2}\right) + i \left(- \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2}\right) + i \left(- \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} + \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2}\right) + i \left(\frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} + \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2}\right)
    произведение
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    1*|-   /   - - -----  + I*  /   - + ----- |*|  /   - - -----  - I*  /   - + ----- |*|-   /   - + -----  - I*  /   - - ----- |*|  /   - + -----  + I*  /   - - ----- |*|I*|---------------------- + ----------------------| + ---------------------- - ----------------------|*|I*|---------------------- - ----------------------| + ---------------------- + ----------------------|*|I*|---------------------- - ----------------------| - ---------------------- - ----------------------|*|I*|- ---------------------- - ----------------------| + ---------------------- - ----------------------|
      \  \/    2     4        \/    2     4   / \\/    2     4        \/    2     4   / \  \/    2     4        \/    2     4   / \\/    2     4        \/    2     4   / \  \          2                        2           /             2                        2           / \  \          2                        2           /             2                        2           / \  \          2                        2           /             2                        2           / \  \            2                        2           /             2                        2           /
    1(1224+i24+12)(1224i24+12)(24+12i1224)(24+12+i1224)(212242+224+122+i(212242+224+122))(212242+224+122+i(224+122+212242))(224+122212242+i(212242+224+122))(224+122+212242+i(224+122212242))1 \left(- \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}} + i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}\right) \left(\sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}} - i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}\right) \left(- \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}} - i \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}\right) \left(\sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}} + i \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}\right) \left(- \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} + \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + i \left(\frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} + \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2}\right)\right) \left(\frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} + \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + i \left(- \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2}\right)\right) \left(- \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} - \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} + i \left(- \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} + \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2}\right)\right) \left(- \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2} + i \left(- \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} - \frac{\sqrt{2} \sqrt{\frac{1}{2} - \frac{\sqrt{2}}{4}}}{2}\right)\right)
    =
    1
    11
    Численный ответ [src]
    x1 = 0.38268343236509 - 0.923879532511287*i
    x2 = -0.923879532511287 + 0.38268343236509*i
    x3 = 0.923879532511287 - 0.38268343236509*i
    x4 = 0.923879532511287 + 0.38268343236509*i
    x5 = -0.38268343236509 - 0.923879532511287*i
    x6 = -0.38268343236509 + 0.923879532511287*i
    x7 = -0.923879532511287 - 0.38268343236509*i
    x8 = 0.38268343236509 + 0.923879532511287*i
    График
    x^8+1=0 (уравнение) /media/krcore-image-pods/hash/equation/d/5b/d0208e463d583082fdb052f5f2878.png