[tex]\bf f(x)=x^4-13x^2+36\implies \stackrel{f(x)}{0}=x^4-13x^2+36
\\\\\\
0=(x^2)^2-13x^2+36~~
\begin{cases}
-4-9=\stackrel{middle~term}{-13}\\
-4\cdot -9=\stackrel{constant}{36}
\end{cases}
\\\\\\
0=(x^2-4)(x^2-9)\\\\
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0=x^2-4\implies 4=x^2\implies \pm\sqrt{4}=x\implies \pm 2 =x\\\\
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0=x^2-9\implies 9=x^2\implies \pm\sqrt{9}=x\implies \pm 3= x[/tex]
