Solution
= Solution
{c}
Take \b[$Y=X_1+X_2$]. Independence makes the joint exponential expectation factor:
$$
\mathbb Ee^{t(X_1+X_2)}=\mathbb E(e^{tX_1}e^{tX_2})=M_1(t)M_2(t).
$$
This is the product rule for the <moment-generating function> of an independent sum.