Let
Find a formula for $f'(x)$.

Note: $f(x)$ is differentiable at $x=-1$, but is NOT differentiable at $x=1$. Can you see why that is so?

$\text{if} \quad x<-1$
$f'(x)=$ $\text{if} \quad -1 \leq x <1$
$\text{if} \quad x>1$