F(x) = 1 - a^3/x^3 ; x≥a
=0 ; elsewhere
f(x) = F'(x)
=3a^3/x^4 ; x≥a
=0 ; elsewhere
E(X)
= ∫(a->∞) xf(x) dx
= ∫(a->∞) 3a^3/x^3 dx
=-(3/2)a^3.[1/x^2]|(a->∞)
=(3/2)a
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