Find the interval in which the function are strictly increasing or strictly decreasing :

$f\left(x\right)=4x^3-6x^2-72x+30$.

Find the interval in which the function are strictly increasing or strictly decreasing :

$f\left(x\right)=2x^3-9x^2+12x+15$.

Find the interval in which the function are strictly increasing or strictly decreasing :

$f\left(x\right)=-2x^3-9x^2-12x+1$.

Find the interval in which the function are strictly increasing or strictly decreasing :

$f\left(x\right)=x+\frac{1}{x}, x\ne 0$.

Find the interval in which the function are strictly increasing or strictly decreasing :

$f\left(x\right)=\frac{4x^2+1}{x}, x\ne 0$.

Find the interval in which the function are strictly increasing or strictly decreasing :

$f\left(x\right)=x^4-8x^3+22x^2-24x+21$.

Find the interval in which the function are strictly increasing or strictly decreasing :

$f\left(x\right)=\frac{1}{4}{x}^4+\frac{2}{3}{x}^3-\frac{5}{2}{x}^2-6x+9$.

Find the interval in which the function are strictly increasing or strictly decreasing :

$f\left(x\right)=x^4-4x$.