The Student Room Group
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The expectation E[T] is also known as the mean, and is defined for discrete random variables (of which Geometric is one) as E[T]=i=1iP(T=i)E[T] = \sum_{i=1}^{\infty}iP(T=i) . (Really it is defined as the sum over any value the random variable can take times by that value) Note the definition is an integral for continuous random variables.

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