Suppose that $\sum a_n$ and $\sum b_n$ are convergent series with
$a_n>0$ and $b_n>0$. Which {\bf one} of the following four series
is necessarily convergent? (Hint: You might try using the comparison
test, or search for examples among the class of $p$-series.)
\correct $\sum a_nb_n$
\wrong $\sum \frac{a_n}{b_n}$
\wrong $\sum \sqrt{a_n}$
\wrong $\sum \frac{1-a_n}{1+b_n}$
\wrong $\sum\frac1{\sqrt{a_n}}$