[tex]a)\\\\9x^3+18x^2-4x-8=0\\9x^2(x+2)-4(x+2)=0\\(9x^2-4)(x+2)=0\\9x^2-4=0\\9x^2=4 /:9\\x^2=\frac49\\x=\frac23 \text{ v } x=-\frac23\\x+2=0\\x=-2\\\\\text{Odp. } x=\frac23 \text{ v } x=-\frac23 \text{ v } x=-2[/tex]
[tex]b)\\\\-x^2(x-7)(5x^2+4)=0\\-x^2=0\\x=0\\\\x-7=0\\x=7\\\\5x^2+4=0 \\5x^2=-4 /:5\\x^2=-\frac45 -\text{ urojona}\\\\\text{Odp. } x=0 \text{ v } x=7[/tex]
[tex]c)\\\\-x^4+3x^2+4=0\\t=x^2\\-t^2+3t+4=0\\\Delta=3^2-4*(-1)*4=9+16=25\\\sqrt{\Delta}=5\\t_1=\frac{-3-5}{-2}=\frac{-8}{-2}=4\\t_2=\frac{-3+5}{-2}=\frac{2}{-2}=-1\\\\x_1=\sqrt{t_1}=\sqrt4=2 \text{ v } x=-2\\x_2=\sqrt{t_2}=\sqrt{-1}\\\\Odp. x=2 \text{ v } x=-2[/tex]
