het S be the set of points where the function,
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 $x=3]$

$\Sigma f(f(x))=f(f(1)+f(f(3))+f(f(5))$

$=1+1+1=3$