$f(x)=\left{\begin{array}{cc}
x, & x \lt -1 \
x^{21}, & -1 \leq x \lt 0 \
x, & 0 \leq x \lt 1 \
x^{21}, & x \geq 1
\end{array}\right.$
f(x) is continuous everywhere.
$\begin{aligned}
& \therefore \mathrm{n}=0 \
& \mathrm{f}^{\prime}(\mathrm{x})=\left{\begin{array}{cc}
1, & \mathrm{x} \lt -1 \
21 \mathrm{x}^{20}, & -1 \leq \mathrm{x} \lt 0 \
1, & 0 \lt \mathrm{x} \lt 1 \
21 \mathrm{x}^{20}, & \mathrm{x} \geq 1
\end{array}\right.
\end{aligned}$
∴f(x) is non-differentiable at x=−1,0,1
$\begin{aligned}
& \therefore \mathrm{m}=3 \
& \mathrm{~m}+\mathrm{n}=3
\end{aligned}$