7. Let $X_1, X_2, \dots, X_k$ be jointly distribution random variables with joint PDF $f(x_1, x_2, \dots, x_k)$ and let \\
$a, a_1, a_2, \dots, a_k$ be real constants. Use induction to show that,\\
$E[a + a_1X_1 + a_2X_2 + \dots + a_kX_k] = a + a_1E[X_1] + a_2E[X_2] + \dots + a_kE[X_k]$