$\begin{array}{1 1}-\large\frac{\Delta p_1}{p_1^*}=x_1\\-\large\frac{\Delta p_1}{p_2^*}=x_2\\-\large\frac{\Delta p_1}{p_1^*}=x_2\\-\large\frac{\Delta p_1}{p_2^*}=x_1\end{array} $