My first math page

$\begin{matrix} E_{D} & = & \frac{change in Q \div average value of Q}{change in P \div average value of P} \\ = & \frac{(Q_{2} - Q_{1}) \div [(Q_{1} + Q_{2}) \div 2]}{(P_{2} - P_{1}) \div [(P_{1} + P_{2}) \div 2]} \end{matrix}$