离散概率分布期望与方差
aX+b的期望与方差
E(aX+b) = a * E(X)
Var(aX+b) = a * a * Var(X)
独立重复事件的期望与方差(X1 + X2 + ...... + Xn)
E(X1 + X2 + ...... + Xn) = n * E(X)
Var(X1 + X2 + ...... + Xn) = n * Var(X)
独立随机事件的期望与方差(X + Y)
E(X + Y) = E(X) + E(Y)
Var(X + Y) = Var(X) + Var(Y)
E(X - Y) = E(X) - E(Y)
Var(X - Y) = Var(X) + Var(Y)
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