1. First mean value theorem for definite integrals.
https://en.wikipedia.org/wiki/Mean_value_theorem
2. Graduate Studies in Mathematics;RG Bartle,2001
3. Courant, R.: Differential and Integral Calculus, Vol. I. Wiley Classics Library. Wiley, New York (1988). Translated from the German by E. J. McShane, Reprint of the second edition (1937), A Wiley-Interscience Publication
4. Flett, T.M.: A mean value theorem. Math. Gazette 42, 38–39 (1958)
5. Goursat, E.: A Course in Mathematical Analysis: Vol 1: Derivatives and Differentials, Definite Integrals, Expansion in Series, Applications to Geometry. Vol. 2, Part 1: Functions of a Complex Variable. Vol. 2, Part 2: Differential Equations. Translated by E. R. Hedrick (Vol. 1), and E. R. Hedrick and O. Dunkel (Vol. 2). Dover Publications, Inc., New York (1959)