Hi, I'm having a hard time understanding how to determine where a function is not differentiable (the question keeps coming up in my homework though sadly, it didn't in the lesson!) I understand that a point where the function is not differentiable means the derivative does not exist there. However, how can I tell, based off either a graph or equation, when a point isn't differentiable?

For example, I'm currently doing this question:

Sketch the graph of f(x) = |x^2 - 1|

a) For what values of x is f not differentiable?

b) Find a formula for f1 and sketch the graph of f1

c) Find f1 at -2, 0, and 3.

Any help is greatly appreciated!