The function \[ f(x)= \begin{cases} 2x^2-1, & \text{if } 1 \leq x \leq 4 \\ 151-30x, & \text{if } 4 < x \leq 5 \end{cases} \] is not suitable to apply Rolle's theorem since:
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Even if a piecewise function "connects" (is continuous), it can still have a "sharp corner" where the slopes don't match. This corner makes it non-differentiable.