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IMC 2002 Problem 12

    Let f:Rn→Rf:\mathbb{R}^{n}\rightarrow \mathbb{R} be a convex function whose gradient ∇f\nabla f exists at every point of Rn\mathbb{R}^{n} and satisfies the condition ∃L>0∀x1,x2∈Rn:  ∣∣∇f(x1)−∇f(x2)∣∣≤L∣∣x1−x2∣∣.\exists L>0 \forall x_{1},x_{2}\in \mathbb{R}^{n}:\; ||\nabla f(x_{1})-\nabla f(x_{2})||\leq L||x_{1}-x_{2}||. Prove that ∀x1,x2∈Rn:  ∣∣∇f(x1)−∇f(x2)∣∣2≤L⟨∇f(x1)−∇f(x2),x1−x2⟩.\forall x_{1},x_{2}\in \mathbb{R}^{n}:\; ||\nabla f(x_{1})-\nabla f(x_{2})||^{2}\leq L\langle\nabla f(x_{1})-\nabla f(x_{2}), x_{1}-x_{2}\rangle.

    Dataset text AI-MO AoPS. Quoted with source.

    Official text: International Mathematics Competition for University Students 2002 official paper, page 4.

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