The third question is why A and B are nondifferentiable at X=0, as shown in the above figure.

1, the third question a is not derivable at X=0, because it is meaningless to take derivative with composite function, so we should define judgment with derivative, and the function is not derivable at x=0.

2. The third question B is not derivable at X=0, because the dividing point of piecewise function is judged by the definition of left and right derivatives.

Note: however, discontinuity can also be used here to represent non-derivative.

In view of the specific Rolle theorem of higher mathematics, the third questions A and B are not differentiable at X=0, and the detailed process is shown in the above figure.