因为x1是x2的替代品，有
\[
\frac{\partial x_{1}}{\partial p_{2}} > 0
\]
并且正常商品必然有
\[
\frac{\partial p_{2}}{\partial x_{2}} < 0
\]
那么
\[\frac{\partial x_{1}}{\partial x_{2}} < 0\]
因此
\[
\frac{\partial x_{2}}{\partial p_{1}} = \frac{\partial x_{2}}{\partial x_{1}} \frac{\partial x_{1}}{\partial p_{1}} < 0
\]