IS曲线的论述：Equilibrium in the Goods Market
Economists define aggregate expenditure on real GDP as the sum of consumption demand,
C; demand for investment in business plant and equipment, inventories, and housing, I;
government purchases of goods and services, G; and net exports (or exports of goods and
services minus imports of goods and services), NX. So, we can write that aggregate expenditure,
AE, is:AE=C+I+G+NX
Recall that gross domestic product (GDP) is the market value of all final goods and services
produced in a country during a period of time. Nominal GDP is calculated using the
current year’s prices, while real GDP is calculated using the prices in a base year. Because real
GDP gives a good measure of a country’s output, corrected for changes in the price level, it is
the measure of aggregate output that we will use initially. The goods market includes trade in
all final goods and services that the economy produces during a particular period of time—
in other words, all goods that are included in real GDP. Equilibrium occurs in the goods market
when the value of goods and services demanded—aggregate expenditure, AE—equals the
value of goods and services produced—real GDP, Y. So, at equilibrium:  AE=Y
What if aggregate expenditure is less than real GDP? In that case, some goods that
were produced are not sold, and inventories of unsold goods will increase. For example, if
General Motors (GM) produces and ships to dealers 250,000 cars in a particular month
but sells only 225,000, inventories of cars on the lots of GM’s dealers will rise by 25,000
cars. (Notice that because inventories are counted as part of investment, in this situation,
actual investment spending will be greater than planned investment spending.) If the decline
in demand is affecting not just automobiles but other goods and services as well, firms are
AE = Y.
AE = C + I + G + NX.