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An investment in company A has an expected return of $30,000\$ 30,000 with a standard deviation of $4000\$ 4000. An investment in company B has an expected return of $45,000\$ 45,000 with a standard deviation of $3000\$ 3000. If investments are made in both companies, what are the expectation and standard deviation of the total return?

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Let XA\color{#c34632}{X_A} represents the return\textit{the return} if the money is invested in company A, XB\color{#c34632}{X_B} represents the return\textit{the return} if the money is invested in company B, and TT represents the total\textbf{the total} return. Then,

T=XA+XB{\color{Brown}{T=X_A+X_B}}

For these two,clearly, independent\color{#c34632}{\text{independent}} random variables XAX_A and XBX_B, from the task, we have

μA=E(XA)=30000,σA=Var(XA)=4000\mu_A=E(X_A)=30000\,,\,\sigma_A=\sqrt{\mathrm{Var}(X_A)}=4000

μB=E(XB)=45000,σB=Var(XB)=3000\mu_B=E(X_B)=45000\,,\,\sigma_B=\sqrt{\mathrm{Var}(X_B)}=3000

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