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E(X) = c∑ i=1∞pk = c( 1 1−p − 1) = − 1 log (1−p) p 1−p
E(X) = E(X2)−(E(X))2 = c∑ i=1∞ipi−(− 1 log (1−p) p 1−p)2 = − 1 log (1−p) p (p−1)2 −(− 1 log (1−p) p 1−p)2