Q.If $f(x)=\begin{cases}x^2+3,& x\le2\\ 5x+7,& x>2\end{cases}$, then find $f(0)$.

A piecewise function is defined by two or more separate formulas, each valid only on its own specified part of the domain; evaluating it always starts by checking which piece the input falls into (paying close attention to which endpoint each inequality includes — a filled dot on a graph marks an included point, a hollow dot an excluded one). Two named piecewise functions recur constantly.

The signum function, $\text{sgn}(x)=-1$ for $x<0$, $0$ at $x=0$, $+1$ for $x>0$, reduces any real number to just its sign; domain $R$, range the three-point set $\{-1,0,1\}$. …