Do you mean like an inner product (i.e. z_n = x_n * y_n) or do you mean you multiply two whole final values together? (Like z = x * y where x is final value for series x and similar for y_n)?
I'm reviewing Meirovitch's "Methods of Analytical Dynamics," and I don't understand the commutation of the derivative from r to dr:
$$
\mathbf{F} \cdot d\mathbf{r} = m \ddot{\mathbf{r}} \cdot d\mathbf{r} = m\mathbf{\dot{r}} \cdot d\mathbf{\dot{r}}
$$