a)
[tex]4x^4-45x^2+81=0\\t=x^2\\\\4t^2-45t+81=0\\\\delta:2025-4*4*81=792\\\sqrt{729}= 27\\t_1=\frac{45+27}{8}= 9\\t_2=\frac{45-27}{8}= 2,25\\\\x_1=3\\x_2=-3\\x_3=\frac{3}{2}\\ x_4=-\frac{3}{2}[/tex]
b)
[tex]-4x^4+6x^2-2=0\\t=x^2\\\\-4t^2+6t-2=0\\-2t^2+3t-1=0\\\\delta: 9-4*(-2)*(-1)=1\\\sqrt{1}=1\\ t_1=\frac{-3+1}{-4}=\frac{1}{2}\\ t_2=\frac{-3-1}{-4}=1\\ x_1=\frac{\sqrt{2} }{2}\\ x_2=-\frac{\sqrt{2} }{2} \\x_3=1\\x_4=-1[/tex]
c)
[tex]x^4-4x^2+16=0\\t=x^2\\t^2-4t+16=0\\\\delta:16-4*1*16 < 0\\[/tex]
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