a)
[tex]x^4-13x^2+36=0\\(x^2)^2-13x^2+36=0\\x^2=t[/tex]
t∈R
[tex]t^2-13t+36=0\\delta=b^2-4ac=(-13)^2-4*36*1=169-144=25\\\sqrt{delta}=5\\ t_{1} =\frac{-b-\sqrt{delta}}{2a}=\frac{13-5}{2}=4\\t_2=\frac{-b+\sqrt{delta}}{2a}=\frac{13+5}{2}=9\\x^2=4, x^2=9\\x=2, x=-2, x=3, x=-3[/tex]
b)
[tex]x^4-9x^2=0\\x^2=t[/tex]
t∈R
[tex]t^2-9t=0\\t(t-9)=0\\t=0, t=9\\x^2=0, x^2=9\\x=0,x=3, x=-3[/tex]