[tex]x^4 - 28x^2 = 0[/tex]
[tex]x^2(x^2 - 28) = 0[/tex]
[tex]x^2 = 0 \ \ or \ \ (x^2 - 28) = 0[/tex]
[tex]x = 0 \ \ or \ \ x^2 = 28[/tex]
[tex]x = 0 \ \ or \ \ x = \pm \sqrt{28} [/tex]
[tex]x = 0 \ \ or \ \ x = \pm2 \sqrt{7}[/tex]
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Answer : x = 0 or 2√7 or -2√7
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