a² - b² = (a - b)(a + b)
x^2/7 ≠ 2 x ≠ 2^7/2
x > 0
x^8/7 - 4 = (x^4/7)^2 - 2^2 = (x^4/7 - 2)(x^4/7 + 2)
x^4/7 - 4 = (x^2/7)^2 - 2^2 = (x^2/7 - 2)(x^2/7 + 2)
(x^4/7 - 4) / (x^2/7 - 2) = x^4/7 + 2
(x^8/7 - 4) / (x^4/7 + 2) = x^4/7 - 2
(x^8/7 - 4) / (x^4/7 + 2) - (x^4/7 - 4) / (x^2/7 - 2) = 2
x^4/7 - 2 - x^2/7 - 2 - 2 = 0
x^2/7 = t > 0
t^2 - t - 6 = 0
D = 1 + 24 = 25
t12 =(1 +- 5)/2 = -2 3
t = -2 нет
t = 3
x^2/7 = 3
⁷√x² = 3
x = 3^7/2