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what is mathematical expectation of a random variable
In probability theory, the expected value of a random variable is a key aspect of its probability distribution.
what is mathematical expectation of a discrete random variable
The expected value of a discrete random variable is the probability-weighted average of all its possible values.

Question: in other words?
Self-contained rewrite: can you explain about The expected value of a discrete random variable is the probability-weighted average of all its possible values.