Update chapters/functions.tex

Co-authored-by: Elena Shmoylova <eshmoylova@users.noreply.github.com>

chapters/functions.tex

@@ -1416,7 +1416,7 @@ \subsection{Using the Derivative Annotation}\label{using-the-derivative-annotati
 \end{lstlisting}
 In the example above \lstinline!zeroDerivative=y! implies that
 \begin{eqnarray*}
-\frac{d}{dt}f(x(t),y(t))&=&\frac{\partial f}{\partial x}\frac{dx}{dt}+\frac{\partial f}{\partial y}\frac{dy}{dt}+\frac{\partial f}{\partial o}\frac{do}{dt}\\
+\frac{d}{dt}f(x(t),y(t),o(t))&=&\frac{\partial f}{\partial x}\frac{dx}{dt}+\frac{\partial f}{\partial y}\frac{dy}{dt}+\frac{\partial f}{\partial o}\frac{do}{dt}\\
 &=&\frac{\partial f}{\partial x}\frac{dx}{dt}+\frac{\partial f}{\partial y}\cdot 0+\frac{\partial f}{\partial o}\cdot 0\\
 &=&\frac{\partial f}{\partial x}\frac{dx}{dt}
 \end{eqnarray*}