墨西哥和巴西各自的贸易伙伴不同。墨西哥主要与美国贸易，巴西与美国和欧盟的贸易量大致相当。墨西哥的贸易量相对其GDP而言很大。用引力模型解释这种现象。

Mexico and Brazil have very different trading patterns. Mexico trades mainly with the United States，Brazil trades about equally with the United States and with the European Union; Mexico does much more trade relative to its GDP.Explain these differences using the gravity model，

用高斯消去法求解下述线性力程组：

用乔列斯基分解计算下述线性方程组:

假如制造业工人收入少于其他行业工人的收入，如果对制造业产品征收大量关税，那么在经济中会对真实收入分配产生什么样的影响?

Suppose that workers involved in manufacturing are paid less than all other workers in the economy. What would be the effect on the real income distribution within the economy if there were a substantial tariff levied on manufactured goods?