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Prove this integral inequality

Source: 2019 Jozsef Wildt International Math Competition-W. 31

May 19, 2020
integrationinequalitiescalculus

Problem Statement

Let a,bΓa, b \in \Gamma, a<ba < b and the differentiable function f:[a,b]Γf : [a, b] \to \Gamma, such that f(a)=af (a) = a and f(b)=bf (b) = b. Prove that ab(f(x))2dxba\int \limits_{a}^{b} \left(f'(x)\right)^2dx \geq b-a