Question:
het $S$ be the set of points where the function,
$(x)=|2-| x-3||, x \in \boldsymbol{R}$, is not differentiable.
hen $\sum_{x \in S} f(f(x))$ is equal to
Solution:
$f(x)$ is non differentiable at $x=1,3,5$
$[\because|x-3|$ is not differentiable at $x=3]$
$\Sigma f(f(x))=f(f(1)+f(f(3))+f(f(5))$
$=1+1+1=3$