Integral mean value theorem

Let f:[a,b]→R be Riemann integrable. Let

m:=infx∈[a,b]f(x),M:=supx∈[a,b]f(x).

Then there exists a number μ∈[m,M] such that

∫abf(x)dx=μ(b−a).

If, in addition, f has the Darboux property (intermediate value property; for example if f is continuous or if f is a derivative), then there exists ξ∈[a,b] such that

∫abf(x)dx=f(ξ)(b−a).