虽然我们在本章中建立的凯恩斯交叉图假设税收是一个固定量，但在许多国家(包括美国)税收取决于收入。我们用下式写出税收收入，以代表税制： 和t为税收的参数；参数t为边际税率：如果收入增加1美元，税收增加t×1美元。 a．这种税制如何改变消费对GDP变动做出反应的方式? b．在凯恩斯交叉图中。这种税制如何改变政府购买乘数? c．在IS-LM模型中，这种税制如何改变IS曲线的斜率? Although our development of the Keynesian cross in this chapter assumes that taxes are a fixed amount, in many countries (including the United States) taxes depend on income. Let's represent the tax system by writing tax revenue as  where  and t are parameters of the tax code. The parameter t is the marginal tax rate: if income rises by $1, taxes rise by t×$1. a．How does this tax system change the way consumption responds to changes in GDP? b．In the Keynesian cross, how does this tax system alter the government-purchases multiplier? c．In the IS-LM model, how does this tax system alter the slope of the IS curve?