## Calculus (3rd Edition)

$x=0,\ \ \ x=2,\ \ \ x=-2$ $f (x)$ is decreasing for $x\lt -1.4$ and $0\lt x \lt 1.4$
We are given $$f(x)=x^{4}-4 x^{2}$$ We factor and set the function equal to zero to find the roots: \begin{align*} x^{4}-4 x^{2}&=0\\ x^2(x^2-4)&=0\\ x^2(x-2)(x+2)&=0\end{align*} Thus, the roots are $$x=0,\ \ \ x=2,\ \ \ x=-2$$ We use the figure shown below to find where $f (x)$ is decreasing: $x\lt -1.4$ and $0\lt x \lt 1.4$