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Is this an abuse of Rolle's theorem?
Rolle's theorem
If a real-valued function f is continuous on a proper closed interval [a, b], differentiable on the open interval (a, b), and f(a) = f(b), then there exists at least one c in the open interval (a, b) such that f'(c) = 0.
##[x_1, x_1]##, which means ##x_1\leq x\leq x_1##, is not an interval but a point. And ##(x_1, x_1)##, which means ##x_1<x<x_1##, doesn't make sense. So how can we apply Rolle's theorem?