5.2 The Definite Integral - Calculus Volume 1 | OpenStax
https://openstax.org/books/calculus-volume-1/pages/5-2-the-definite-integral
WebDefinition. If f ( x) is a function defined on an interval [ a, b], the definite integral of f from a to b is given by. ∫ a b f ( x) d x = lim n → ∞ ∑ i = 1 n f ( x i *) Δ x, (5.8) provided the limit exists. If this limit exists, the function f ( x) is said to be integrable on [ a, b], or is an integrable function.
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