Monotonicity

If $f\left(x\right)=x^3+b{x}^2+cx+d$ and $0<b^2<c$, then in $(-\infty ,\infty )$

The function $f\left(x\right)=\sin x-kx-c$, where $k$ and $c$ are constants, decreases always when

The interval in which the function $f\left(x\right)=\frac{4x^2+1}{x}$ is strictly decreasing is