Подробное решение
Дано уравнение
$$x^{8} + 1 = 0$$
Т.к. степень в уравнении равна = 8 и свободный член = -1 < 0,
зн. действительных решений у соответствующего уравнения не существует
Остальные 8 корня(ей) являются комплексными.
сделаем замену:
$$z = x$$
тогда уравнение будет таким:
$$z^{8} = -1$$
Любое комплексное число можно представить так:
$$z = r e^{i p}$$
подставляем в уравнение
$$r^{8} e^{8 i p} = -1$$
где
$$r = 1$$
- модуль комплексного числа
Подставляем r:
$$e^{8 i p} = -1$$
Используя формулу Эйлера, найдём корни для p
$$i \sin{\left(8 p \right)} + \cos{\left(8 p \right)} = -1$$
значит
$$\cos{\left(8 p \right)} = -1$$
и
$$\sin{\left(8 p \right)} = 0$$
тогда
$$p = \frac{\pi N}{4} + \frac{\pi}{8}$$
где N=0,1,2,3,...
Перебирая значения N и подставив p в формулу для z
Значит, решением будет для z:
$$z_{1} = - \sqrt{- \frac{\sqrt{2}}{4} + \frac{1}{2}} + i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}$$
$$z_{2} = \sqrt{- \frac{\sqrt{2}}{4} + \frac{1}{2}} - i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}$$
$$z_{3} = - \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}} - i \sqrt{- \frac{\sqrt{2}}{4} + \frac{1}{2}}$$
$$z_{4} = \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}} + i \sqrt{- \frac{\sqrt{2}}{4} + \frac{1}{2}}$$
$$z_{5} = - \frac{\sqrt{2} \sqrt{- \frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} i \sqrt{- \frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2}$$
$$z_{6} = \frac{\sqrt{2} \sqrt{- \frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} - \frac{\sqrt{2} i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} i \sqrt{- \frac{\sqrt{2}}{4} + \frac{1}{2}}}{2}$$
$$z_{7} = - \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} - \frac{\sqrt{2} \sqrt{- \frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} - \frac{\sqrt{2} i \sqrt{- \frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2}$$
$$z_{8} = - \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} \sqrt{- \frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} - \frac{\sqrt{2} i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} - \frac{\sqrt{2} i \sqrt{- \frac{\sqrt{2}}{4} + \frac{1}{2}}}{2}$$
делаем обратную замену
$$z = x$$
$$x = z$$
Тогда, окончательный ответ:
$$x_{1} = - \sqrt{- \frac{\sqrt{2}}{4} + \frac{1}{2}} + i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}$$
$$x_{2} = \sqrt{- \frac{\sqrt{2}}{4} + \frac{1}{2}} - i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}$$
$$x_{3} = - \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}} - i \sqrt{- \frac{\sqrt{2}}{4} + \frac{1}{2}}$$
$$x_{4} = \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}} + i \sqrt{- \frac{\sqrt{2}}{4} + \frac{1}{2}}$$
$$x_{5} = - \frac{\sqrt{2} \sqrt{- \frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} i \sqrt{- \frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2}$$
$$x_{6} = \frac{\sqrt{2} \sqrt{- \frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} - \frac{\sqrt{2} i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} i \sqrt{- \frac{\sqrt{2}}{4} + \frac{1}{2}}}{2}$$
$$x_{7} = - \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} - \frac{\sqrt{2} \sqrt{- \frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} - \frac{\sqrt{2} i \sqrt{- \frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2}$$
$$x_{8} = - \frac{\sqrt{2} \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} + \frac{\sqrt{2} \sqrt{- \frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} - \frac{\sqrt{2} i \sqrt{\frac{\sqrt{2}}{4} + \frac{1}{2}}}{2} - \frac{\sqrt{2} i \sqrt{- \frac{\sqrt{2}}{4} + \frac{1}{2}}}{2}$$
Сумма и произведение корней
[src]
___________ ___________ ___________ ___________ ___________ ___________ ___________ ___________ / ___________ ___________\ / ___________ ___________\ / ___________ ___________\ / ___________ ___________\ / ___________ ___________\ / ___________ ___________\ / ___________ ___________\ / ___________ ___________\
/ ___ / ___ / ___ / ___ / ___ / ___ / ___ / ___ ___ | / ___ / ___ | ___ | / ___ / ___ | ___ | / ___ / ___ | ___ | / ___ / ___ | ___ | / ___ / ___ | ___ | / ___ / ___ | ___ | / ___ / ___ | ___ | / ___ / ___ |
\/ 2 - \/ 2 I*\/ 2 + \/ 2 \/ 2 - \/ 2 I*\/ 2 + \/ 2 \/ 2 + \/ 2 I*\/ 2 - \/ 2 \/ 2 + \/ 2 I*\/ 2 - \/ 2 \/ 2 *\\/ 2 + \/ 2 - \/ 2 - \/ 2 / I*\/ 2 *\\/ 2 + \/ 2 + \/ 2 - \/ 2 / \/ 2 *\\/ 2 + \/ 2 + \/ 2 - \/ 2 / I*\/ 2 *\\/ 2 - \/ 2 - \/ 2 + \/ 2 / \/ 2 *\- \/ 2 + \/ 2 - \/ 2 - \/ 2 / I*\/ 2 *\\/ 2 + \/ 2 - \/ 2 - \/ 2 / \/ 2 *\\/ 2 - \/ 2 - \/ 2 + \/ 2 / I*\/ 2 *\- \/ 2 + \/ 2 - \/ 2 - \/ 2 /
- -------------- + ---------------- + -------------- - ---------------- + - -------------- - ---------------- + -------------- + ---------------- + --------------------------------------- + ----------------------------------------- + --------------------------------------- + ----------------------------------------- + ----------------------------------------- + ----------------------------------------- + --------------------------------------- + -------------------------------------------
2 2 2 2 2 2 2 2 4 4 4 4 4 4 4 4
$$\left(- \frac{\sqrt{- \sqrt{2} + 2}}{2} + \frac{i \sqrt{\sqrt{2} + 2}}{2}\right) + \left(\frac{\sqrt{- \sqrt{2} + 2}}{2} - \frac{i \sqrt{\sqrt{2} + 2}}{2}\right) + \left(- \frac{\sqrt{\sqrt{2} + 2}}{2} - \frac{i \sqrt{- \sqrt{2} + 2}}{2}\right) + \left(\frac{\sqrt{\sqrt{2} + 2}}{2} + \frac{i \sqrt{- \sqrt{2} + 2}}{2}\right) + \left(\frac{\sqrt{2} \left(- \sqrt{- \sqrt{2} + 2} + \sqrt{\sqrt{2} + 2}\right)}{4} + \frac{\sqrt{2} i \left(\sqrt{- \sqrt{2} + 2} + \sqrt{\sqrt{2} + 2}\right)}{4}\right) + \left(\frac{\sqrt{2} \left(\sqrt{- \sqrt{2} + 2} + \sqrt{\sqrt{2} + 2}\right)}{4} + \frac{\sqrt{2} i \left(- \sqrt{\sqrt{2} + 2} + \sqrt{- \sqrt{2} + 2}\right)}{4}\right) + \left(\frac{\sqrt{2} \left(- \sqrt{\sqrt{2} + 2} - \sqrt{- \sqrt{2} + 2}\right)}{4} + \frac{\sqrt{2} i \left(- \sqrt{- \sqrt{2} + 2} + \sqrt{\sqrt{2} + 2}\right)}{4}\right) + \left(\frac{\sqrt{2} \left(- \sqrt{\sqrt{2} + 2} + \sqrt{- \sqrt{2} + 2}\right)}{4} + \frac{\sqrt{2} i \left(- \sqrt{\sqrt{2} + 2} - \sqrt{- \sqrt{2} + 2}\right)}{4}\right)$$
/ ___________ ___________\ / ___________ ___________\ / ___________ ___________\ / ___________ ___________\ / ___________ ___________\ / ___________ ___________\ / ___________ ___________\ / ___________ ___________\
___ | / ___ / ___ | ___ | / ___ / ___ | ___ | / ___ / ___ | ___ | / ___ / ___ | ___ | / ___ / ___ | ___ | / ___ / ___ | ___ | / ___ / ___ | ___ | / ___ / ___ |
\/ 2 *\\/ 2 + \/ 2 + \/ 2 - \/ 2 / \/ 2 *\\/ 2 + \/ 2 - \/ 2 - \/ 2 / \/ 2 *\\/ 2 - \/ 2 - \/ 2 + \/ 2 / \/ 2 *\- \/ 2 + \/ 2 - \/ 2 - \/ 2 / I*\/ 2 *\\/ 2 + \/ 2 + \/ 2 - \/ 2 / I*\/ 2 *\\/ 2 + \/ 2 - \/ 2 - \/ 2 / I*\/ 2 *\\/ 2 - \/ 2 - \/ 2 + \/ 2 / I*\/ 2 *\- \/ 2 + \/ 2 - \/ 2 - \/ 2 /
--------------------------------------- + --------------------------------------- + --------------------------------------- + ----------------------------------------- + ----------------------------------------- + ----------------------------------------- + ----------------------------------------- + -------------------------------------------
4 4 4 4 4 4 4 4
$$\frac{\sqrt{2} \left(- \sqrt{\sqrt{2} + 2} - \sqrt{- \sqrt{2} + 2}\right)}{4} + \frac{\sqrt{2} \left(- \sqrt{\sqrt{2} + 2} + \sqrt{- \sqrt{2} + 2}\right)}{4} + \frac{\sqrt{2} \left(- \sqrt{- \sqrt{2} + 2} + \sqrt{\sqrt{2} + 2}\right)}{4} + \frac{\sqrt{2} \left(\sqrt{- \sqrt{2} + 2} + \sqrt{\sqrt{2} + 2}\right)}{4} + \frac{\sqrt{2} i \left(- \sqrt{\sqrt{2} + 2} - \sqrt{- \sqrt{2} + 2}\right)}{4} + \frac{\sqrt{2} i \left(- \sqrt{\sqrt{2} + 2} + \sqrt{- \sqrt{2} + 2}\right)}{4} + \frac{\sqrt{2} i \left(- \sqrt{- \sqrt{2} + 2} + \sqrt{\sqrt{2} + 2}\right)}{4} + \frac{\sqrt{2} i \left(\sqrt{- \sqrt{2} + 2} + \sqrt{\sqrt{2} + 2}\right)}{4}$$
___________ ___________ ___________ ___________ ___________ ___________ ___________ ___________ / ___________ ___________\ / ___________ ___________\ / ___________ ___________\ / ___________ ___________\ / ___________ ___________\ / ___________ ___________\ / ___________ ___________\ / ___________ ___________\
/ ___ / ___ / ___ / ___ / ___ / ___ / ___ / ___ ___ | / ___ / ___ | ___ | / ___ / ___ | ___ | / ___ / ___ | ___ | / ___ / ___ | ___ | / ___ / ___ | ___ | / ___ / ___ | ___ | / ___ / ___ | ___ | / ___ / ___ |
\/ 2 - \/ 2 I*\/ 2 + \/ 2 \/ 2 - \/ 2 I*\/ 2 + \/ 2 \/ 2 + \/ 2 I*\/ 2 - \/ 2 \/ 2 + \/ 2 I*\/ 2 - \/ 2 \/ 2 *\\/ 2 + \/ 2 - \/ 2 - \/ 2 / I*\/ 2 *\\/ 2 + \/ 2 + \/ 2 - \/ 2 / \/ 2 *\\/ 2 + \/ 2 + \/ 2 - \/ 2 / I*\/ 2 *\\/ 2 - \/ 2 - \/ 2 + \/ 2 / \/ 2 *\- \/ 2 + \/ 2 - \/ 2 - \/ 2 / I*\/ 2 *\\/ 2 + \/ 2 - \/ 2 - \/ 2 / \/ 2 *\\/ 2 - \/ 2 - \/ 2 + \/ 2 / I*\/ 2 *\- \/ 2 + \/ 2 - \/ 2 - \/ 2 /
- -------------- + ---------------- * -------------- - ---------------- * - -------------- - ---------------- * -------------- + ---------------- * --------------------------------------- + ----------------------------------------- * --------------------------------------- + ----------------------------------------- * ----------------------------------------- + ----------------------------------------- * --------------------------------------- + -------------------------------------------
2 2 2 2 2 2 2 2 4 4 4 4 4 4 4 4
$$\left(- \frac{\sqrt{- \sqrt{2} + 2}}{2} + \frac{i \sqrt{\sqrt{2} + 2}}{2}\right) * \left(\frac{\sqrt{- \sqrt{2} + 2}}{2} - \frac{i \sqrt{\sqrt{2} + 2}}{2}\right) * \left(- \frac{\sqrt{\sqrt{2} + 2}}{2} - \frac{i \sqrt{- \sqrt{2} + 2}}{2}\right) * \left(\frac{\sqrt{\sqrt{2} + 2}}{2} + \frac{i \sqrt{- \sqrt{2} + 2}}{2}\right) * \left(\frac{\sqrt{2} \left(- \sqrt{- \sqrt{2} + 2} + \sqrt{\sqrt{2} + 2}\right)}{4} + \frac{\sqrt{2} i \left(\sqrt{- \sqrt{2} + 2} + \sqrt{\sqrt{2} + 2}\right)}{4}\right) * \left(\frac{\sqrt{2} \left(\sqrt{- \sqrt{2} + 2} + \sqrt{\sqrt{2} + 2}\right)}{4} + \frac{\sqrt{2} i \left(- \sqrt{\sqrt{2} + 2} + \sqrt{- \sqrt{2} + 2}\right)}{4}\right) * \left(\frac{\sqrt{2} \left(- \sqrt{\sqrt{2} + 2} - \sqrt{- \sqrt{2} + 2}\right)}{4} + \frac{\sqrt{2} i \left(- \sqrt{- \sqrt{2} + 2} + \sqrt{\sqrt{2} + 2}\right)}{4}\right) * \left(\frac{\sqrt{2} \left(- \sqrt{\sqrt{2} + 2} + \sqrt{- \sqrt{2} + 2}\right)}{4} + \frac{\sqrt{2} i \left(- \sqrt{\sqrt{2} + 2} - \sqrt{- \sqrt{2} + 2}\right)}{4}\right)$$
$$1$$
___________ ___________
/ ___ / ___
\/ 2 - \/ 2 I*\/ 2 + \/ 2
x_1 = - -------------- + ----------------
2 2
$$x_{1} = - \frac{\sqrt{- \sqrt{2} + 2}}{2} + \frac{i \sqrt{\sqrt{2} + 2}}{2}$$
___________ ___________
/ ___ / ___
\/ 2 - \/ 2 I*\/ 2 + \/ 2
x_2 = -------------- - ----------------
2 2
$$x_{2} = \frac{\sqrt{- \sqrt{2} + 2}}{2} - \frac{i \sqrt{\sqrt{2} + 2}}{2}$$
___________ ___________
/ ___ / ___
\/ 2 + \/ 2 I*\/ 2 - \/ 2
x_3 = - -------------- - ----------------
2 2
$$x_{3} = - \frac{\sqrt{\sqrt{2} + 2}}{2} - \frac{i \sqrt{- \sqrt{2} + 2}}{2}$$
___________ ___________
/ ___ / ___
\/ 2 + \/ 2 I*\/ 2 - \/ 2
x_4 = -------------- + ----------------
2 2
$$x_{4} = \frac{\sqrt{\sqrt{2} + 2}}{2} + \frac{i \sqrt{- \sqrt{2} + 2}}{2}$$
/ ___________ ___________\ / ___________ ___________\
___ | / ___ / ___ | ___ | / ___ / ___ |
\/ 2 *\\/ 2 + \/ 2 - \/ 2 - \/ 2 / I*\/ 2 *\\/ 2 + \/ 2 + \/ 2 - \/ 2 /
x_5 = --------------------------------------- + -----------------------------------------
4 4
$$x_{5} = \frac{\sqrt{2} \left(- \sqrt{- \sqrt{2} + 2} + \sqrt{\sqrt{2} + 2}\right)}{4} + \frac{\sqrt{2} i \left(\sqrt{- \sqrt{2} + 2} + \sqrt{\sqrt{2} + 2}\right)}{4}$$
/ ___________ ___________\ / ___________ ___________\
___ | / ___ / ___ | ___ | / ___ / ___ |
\/ 2 *\\/ 2 + \/ 2 + \/ 2 - \/ 2 / I*\/ 2 *\\/ 2 - \/ 2 - \/ 2 + \/ 2 /
x_6 = --------------------------------------- + -----------------------------------------
4 4
$$x_{6} = \frac{\sqrt{2} \left(\sqrt{- \sqrt{2} + 2} + \sqrt{\sqrt{2} + 2}\right)}{4} + \frac{\sqrt{2} i \left(- \sqrt{\sqrt{2} + 2} + \sqrt{- \sqrt{2} + 2}\right)}{4}$$
/ ___________ ___________\ / ___________ ___________\
___ | / ___ / ___ | ___ | / ___ / ___ |
\/ 2 *\- \/ 2 + \/ 2 - \/ 2 - \/ 2 / I*\/ 2 *\\/ 2 + \/ 2 - \/ 2 - \/ 2 /
x_7 = ----------------------------------------- + -----------------------------------------
4 4
$$x_{7} = \frac{\sqrt{2} \left(- \sqrt{\sqrt{2} + 2} - \sqrt{- \sqrt{2} + 2}\right)}{4} + \frac{\sqrt{2} i \left(- \sqrt{- \sqrt{2} + 2} + \sqrt{\sqrt{2} + 2}\right)}{4}$$
/ ___________ ___________\ / ___________ ___________\
___ | / ___ / ___ | ___ | / ___ / ___ |
\/ 2 *\\/ 2 - \/ 2 - \/ 2 + \/ 2 / I*\/ 2 *\- \/ 2 + \/ 2 - \/ 2 - \/ 2 /
x_8 = --------------------------------------- + -------------------------------------------
4 4
$$x_{8} = \frac{\sqrt{2} \left(- \sqrt{\sqrt{2} + 2} + \sqrt{- \sqrt{2} + 2}\right)}{4} + \frac{\sqrt{2} i \left(- \sqrt{\sqrt{2} + 2} - \sqrt{- \sqrt{2} + 2}\right)}{4}$$
x1 = -0.38268343236509 + 0.923879532511287*i
x2 = -0.38268343236509 - 0.923879532511287*i
x3 = 0.923879532511287 - 0.38268343236509*i
x4 = 0.923879532511287 + 0.38268343236509*i
x5 = 0.38268343236509 + 0.923879532511287*i
x6 = -0.923879532511287 - 0.38268343236509*i
x7 = -0.923879532511287 + 0.38268343236509*i
x8 = 0.38268343236509 - 0.923879532511287*i
x8 = 0.38268343236509 - 0.923879532511287*i