Finding the Measure of A, B: Prove m(A)+m(B)=m(AuB)+m(AnB)

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SUMMARY

The discussion focuses on proving the equation m(A) + m(B) = m(A ∪ B) + m(A ∩ B) where A and B are elements of a sigma algebra and m denotes a finite measure. Participants emphasize the importance of recognizing that measures are additive over countable disjoint unions. A suggested approach involves rewriting sets A and B using their intersections to facilitate the proof. The assumption that m is a finite measure is also highlighted as critical to the solution.

PREREQUISITES
  • Understanding of sigma algebras and their properties
  • Knowledge of finite measures and their characteristics
  • Familiarity with set operations, particularly unions and intersections
  • Concept of countable disjoint unions in measure theory
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  • Study the properties of sigma algebras in measure theory
  • Learn about finite measures and their applications
  • Explore the concept of additivity in measures, particularly in countable contexts
  • Investigate examples of disjoint unions and their implications in measure calculations
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Mathematics students, particularly those studying measure theory, and educators looking for insights into teaching measure properties and proofs.

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Homework Statement



A, B in a sigma algebra

Prove
m(A)+m(B)=m(AuB)+m(AnB)

m denotes the measure.

The Attempt at a Solution



Don't see how to do it.

Somehow we are dealing with each individual set and taking the measure on them. Then finding what they equate to.
 
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A measure is additive on countable disjoint unions. So have you tried to write A[itex]\cup[/itex]B as a union of disjoint sets?

(Note: I'm assuming m is a finite measure, i.e. does not take on infinity.)
 
Last edited:
Ok you haven't tried hard Pivoxa
Hint:
use [tex]A= [A - (A \cap B)] \cup (A \cap B) \hspace{10pt}\text{or}\hspace{10pt} B= [B - (A \cap B)] \cup (A \cap B)[/tex] and see what you get from it.
 

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