Add offset in formula

chapters/functions.tex

@@ -1370,7 +1370,7 @@ \subsection{Using the Derivative Annotation}\label{using-the-derivative-annotati
 function f "Simple table lookup"
   input Real x;
   input Real y[:, 2];
-  input Real offset;
+  input Real offset "Shortened to o below";
   output Real z;
 algorithm
   $\ldots$
@@ -1416,8 +1416,8 @@ \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 x}\frac{dx}{dt}+\frac{\partial f}{\partial y}\cdot 0\\
+\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{\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*}
 \end{nonnormative}