自20世纪70年代以来，美国边际税率的下降使得延长工作时间更有利可图。

Falls in marginal tax rates in America since the 1970s have made it all the more profitable to work longer.

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影响边际成本曲线的外部经济，对于得出一条负斜率的行业供给曲线是非常重要的——图5.11即以这种情况说明上面相同的观点。

Figure 5.11 illustrates the same point for the case in which external economies affecting marginal cost curves are sufficiently important to yield a negatively sloping supply curve to the industry.

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由此导致的边际成本曲线下移，可能与图5.7所示的单个厂商准供给曲线为正斜率的情况相一致；

The consequent decline in the marginal cost curves may be consistent with a positively sloped quasi-supply curve of the individual firm as in Figures 5.7(a) and (b);

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由此导致的边际成本曲线下移，可能与图5.7所示的单个厂商准供给曲线为正斜率的情况相一致；

The consequent decline in the marginal cost curves may be consistent with a positively sloped quasi-supply curve of the individual firm as in Figures 5.7(a) and (b);

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