點解LM curve 係upward sloping ?

圖片參考：http://img.sparknotes.com/figures/2/27ecf3a3be058b371903d45b59973dfc/islmgraph.gif

IS-LM模型是由希克斯和漢森於1937年提出用於解釋凱恩斯的宏觀經濟學理論思想的一個模型。凱恩斯認為政府行為可以有效地干預經濟發展。
在後面的幾幅圖中，橫軸Y指的是國民收入或者實際國內生產總值（GDP），而縱軸X指的是利息率,而其中的 IS 曲線指的是I-S,即投資-儲蓄（Investment-Saving） 按照宏觀經濟學的理論，在一般均衡情況下，投資是等於儲蓄的。相當於說，某人存儲的1000元現金就轉化某公司的投資額。IS 曲線是向下傾斜的，表示在某個時候，假如政府投資不變, 國民收入 Y 增加，人們可用於儲蓄的資金增加，那麼市場上的利息率就會下降（這是靜態分析，如果在長期內，IS 曲線的走勢將更複雜）。
其中的 LM 曲線指的是流動性偏好-貨幣供給（Liquidity preference-Money supply），也就是國家的中央銀行可以提供的貨幣量。一般來說，政府為了流通的方便及防止貨幣值的變化，會在國民收入 Y 增加的時候也增加貨幣供給。所以 LM 曲線是向上傾斜的。
In economics, aggregate supply is the total supply of goods and services by a national economy during a specific time period.

aggregate demand is the total demand for goods and services in the economy (Y) during a specific time period. An aggregate demand curve is the sum of individual demand curves. The aggregate demand function is represented as : Yd= C+I+G+NX. This function shows that the aggregate demand is equal to the sum of consumption (C), Investment (I), Government spending (G) and the Net export (NX).

IS curve is the sets of equilibrium national incomes at different interest rate levels, under product market equilibrium.

LM curve is the sets of equilibrium interest rates at different national income levels, under money market equilibrium.

A reduction in interest rate will lead to an increase in asset demand for money. Given the constant money supply, the transaction demand for money falls and through a multiplier process, income falls. Thus LM curve is upward sloping.

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