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1·To derive the horizontal summation , one would take a price and add the total quantity that would be purchased at that price by all buyers.

为了获得横向叠加的结果，需要确定一个价格并且将在那个价格上所有购买者要购买的总数量加起来。

2·This is sometimes called a “horizontal sum”because the summation is over the quantities for each price.

市场需求有时也被称为“水平需求”，因为它是每一价格水平下需求量的加总。