# 供應同需求

## 市場均衡

• 當一件貨品服務嘅價格低過佢嘅均衡點嗰陣，需求量會大過供應量，所以會出現短缺
• 而當件貨品服務嘅價格高過均衡點嗰陣，供應量會大過需求量，所以會出現盈餘

## 彈性

${\displaystyle e_{\langle p\rangle }={\frac {\mathrm {d} Q/Q}{\mathrm {d} P/P}}}$

## 註釋

1. 順帶一提，如果家陣有兩件貨品，消費者對兩者嘅需求會一齊升或者一齊跌，噉呢兩件貨品就算係所謂嘅互補品（complementary）；例子有牙刷牙膏
2. 兩件貨品 ${\displaystyle A}$ ${\displaystyle B}$  係互補品定替代品可以用以下呢條式衡量：
${\displaystyle CG={\frac {\Pr(A\cap B)}{\Pr(A\cup B)}}\,}$
${\displaystyle \Pr(A\cap B)}$  係「消費者買嗮兩件貨」嘅機率，而 ${\displaystyle \Pr(A\cup B)}$  係「消費者買嗮兩件或者淨係買其中一件」嘅機率，如果 ${\displaystyle A}$ ${\displaystyle B}$  係互補品，${\displaystyle CG}$  數值會大，而如果 ${\displaystyle A}$ ${\displaystyle B}$  係替代品，${\displaystyle CG}$  數值會細。

## 攷

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