# Need math help with Upper and lower sums

**Question description**

Let g:[a,b] -> R be a function and suppose g is Riemann integrable. Prove that for every partition P of [a,b], there exists a k in R such that L(f,P) <= k <= U(f,P) and __k is unique__. L(f,P) is the lower sum. U stands for upper sum.

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