[tex]\displaystyle 1)x^4-5x^2+4=0\\t^2-5t+4=0\\t=1,t=4\\x^2=1,x^2=4\\x_1=-1,x_2=1,x_3=-2,x_3=2\\\\2)x^4+5x^2+4=0\\x \notin R\\\\3)2x^4-x^2-6=0\\2t^2-t-6=0\\\\t=-\frac{3}{2},t=2\\ \\x^2=-\frac{3}{2},x^2=2\\ x \notin R, x=б\sqrt{2}\\ x_1=-\sqrt{2},x=\sqrt{2} \\\\4)9x^4-6x^2+1=0\\(3x^2-1)^2=0\\3x^2-1=0\\3x^2=1\\\\x^2=\frac{1}{3}\\ \\x_1=-\frac{\sqrt{3} }{3},x_2=\frac{\sqrt{3} }{3}[/tex]