Form 4 econ

只要係good,咁個marginal benefit必然會係positive

但如果係bad,咁就會係negative

而neuter就係zero,因為數量多少都好其實係對你冇影響

點derive其實好簡單,我諗你應該知道咩叫做marginal use value

唔知唔緊要,其實就係the maximum amount of another good willing to give up, to obtain a unit of that good

簡單啲講其實姐係你覺得樣野既價值係幾多

不過佢都會隨住你擁有既數量,擁有得越多marginal use value就會越低

而呢一野就係the law of diminishing marginal use value講既

假如你已經學左,你就會知我係咩意思

當marginal use value > price

marginal benefit就會係positive

你係唔會停手,你會繼續買,因為你仲會有gain (benefit)

你會買到一點就係price = marginal use value

咁你就停啦,因為marginal benefit = 0

你唔會再買落去,因為之後marginal benefit將會係negative,姐係你會蝕左

唔同既price時,我買既數量將會唔同

咁你就可以見到,例如我$4時我買4件就停手,$3時我買6件就停手

你只要連埋咁多點,你就可以畫到一條demand curve

希望幫到你啦
圖片參考：http://i225.photobucket.com/albums/dd54/icon_2kf/00015.gif