3.
[tex]a) \ f(x) = x^{6} - 8\\\\x^{6}-8 = 0\\\\x^{6} = 8\\\\(x^{2})^{3} = 2^{3}\\\\x^{2} = 2\\\\\underline{x = -\sqrt2 \ \vee \ x = \sqrt2}[/tex]
[tex]b) \ f(x) = 81-x^{8}\\\\81-x^{8} = 0\\\\-x^{8} = -81 \ \ /:(-1)\\\\x^{8} = 81\\\\(x^{2})^{4} = 3^{4}\\\\x^{2} = 3\\\\\underline{x = -\sqrt{3} \ \vee \ x = \sqrt{3}}[/tex]
