fix eqn 1

Author: menon-karthik

docs/pages/add_block.md

@@ -94,6 +94,8 @@ Below are details on the steps required to implement a new block in svZeroDSolve
 
 * Assume a block has the following non-linear governing equations:
 
+\f$ a \frac{dQ_{in}}{dt} + b P_{in} + c \frac{dP_{in}}{dt} Q_{in} + d = 0 \f$
+
 \f$ e \frac{dP_{out}}{dt} + f Q_{out} Q_{out} + g P_{out} + h I_{1} = 0 \f$
 
   * For this block, \f$P_{in}\f$ and \f$Q_{in}\f$ are the pressure and flow at the inlet respectively, \f$P_{out}\f$ and \f$Q_{out}\f$ are the pressure and flow at the outlet, and \f$I_{1}\f$ is an internal variable.