>Chapter 21: The Theory of Consumer Choice

hệ số góc càng lớn càng dốc, càng bé thoải hơn, shift là dịch song song, pivot chếch về
phía nào
I.​ The Budget Constraint (Ràng buộc ngân sách)
●​ Trade- offs:
●​ Budget constraint: The limit on the consumption bundles that a consumer can
afford (Giới hạn về các gói tiêu dùng mà người tiêu dùng có thể chi trả)
●​ income = Qa×Pa + Qb×Pb ( BC)
●​ Slope: tỷ lệ trao đổi theo giá cả thị trường ( downward sloping) < 0 =
𝑟𝑖𝑠𝑒 ( 𝑡ă𝑛𝑔 𝑙ê𝑛 𝑡ℎ𝑒𝑜 𝑐ℎ𝑖ề𝑢 𝑑ọ𝑐) 𝑃𝑥
𝑟𝑢𝑛 ( 𝑐ℎ𝑖ề𝑢 𝑛𝑔𝑎𝑛𝑔)
=- 𝑃𝑦
●​ Changes to the budget constraint:
+​ Income:
○​ A fall in income shifts the budget constraint inward.
○​ An increase in income shifts the budget constraint outward.
+​ Price:
○​ An increase in the price of one good pivots the budget
constraint inward.
○​ A decrease in the price of a good will pivot the budget
constraint outward.
II.​ Indifference Curves ( đường cong bàng quang)
●​ Indifference curve
+​ Curve that shows consumption bundles that give the consumer the
same level of satisfaction (Đường cong biểu thị các gói tiêu dùng
mang lại cho người tiêu dùng mức độ hài lòng như nhau)
●​ MRS = the slope of the indifference curve ( not constant) , từ trái qua phải
MRS giảm dần, the marginal rate of substitution ( tỷ lệ thay thế cận biên)
+​ The rate at which a consumer is willing to trade one good for another
●​ Four properties of Indifference curve
+​ Higher indifference curves are preferred to lower ones.
+​ Indifference curves slope downward ( muốn tăng mặt hàng thứ nhất
thì giảm mặt hàng thứ hai đi )
+​ Indifference curves do not cross (Đường bàng quan không cắt nhau)
+​ Indifference curves are bowed inward (Đường bàng quan cong vào
trong ( cong lồi hướng vào gốc tọa độ).
●​ Trường hợp đặc biệt của đường cong bàng quang:
+​ Shape of an indifference curve
+​ Reveals the consumer’s willingness to trade one good for the
other
+​ Perfect substitutes (sản phẩm thay thế hoàn hảo) :
+​ Two goods with straight-line indifference curves (constant
MRS)
+​ Perfect complements:
+​ Two goods with right-angle ( vuông góc) or L shaped
indifference curves
 +​ Close substitutes & close complements:
+​ Indifference curves for close substitutes are not very bowed.
Ex: hamburgers vs Hotdogs
+​ Indifference curves for close complements are very bowed. Ex:
Shoes vs socks (vớ)
III.​ Optimization ( sự tối ưu)
●​ The consumer’s optimal choices
+​ The best possible combination of the two goods
+​ The combination on the highest possible indifference curve on or
below the budget constraint
●​ Optimum: MRS = relative price
+​ The best bundle of the two goods that the consumer can afford
+​ The point on the budget constraint that touches the highest possible
indifference curve
●​ MRS = Px / Py
+​ Slope of indifference curve = Slope of budget constraint= Px/ Py
IV.​ Changes in Income
●​ Normal good
+​ A good for which an increase in income raises the quantity demanded
●​ Inferior good
+​ A good for which an increase in income reduces the quantity
demanded
●​ A change in income
+​ Shifts the budget constraint
+​ Move on a different indifference curve
V.​ The income and substitution effects
●​ The income effect is the change in consumption that results from the
movement to a new indifference curve. (change in purchasing power tăng
cũng như income tang thi Quantity of normal good tang, Quantity inferior
good giảm VÀ NGƯỢC LẠI )
●​ The substitution effect is the change in consumption that results from
moving to a new point on the same indifference curve with a different
marginal rate of substitution. ( more expensive. Px/ Py tăng thì Qx giảm, Qy
tăng)
●​ substitution effect dominates:
VI.​ Deriving the Demand Curve
●​ The demand curve
+​ Reflects the consumption decisions (Phản ánh các quyết định tiêu
dùng)
+​ Shows the quantity demanded of a good for any given price (Hiển thị
số lượng yêu cầu của một hàng hóa cho bất kỳ mức giá nhất định
nào)
+​ Is a summary of the optimal decisions that arise from the budget
constraint and indifference curves (Là bản tóm tắt các quyết định tối
ưu phát sinh từ hạn chế ngân sách và đường cong thờ ơ )
●​ Application 1: Giffen Goods: not obey law of demand ( khi P tăng thì Q
cũng tăng)
+​ Example: potator ( inferior good) and meat
 ○​ Price of potatoes increase => purchasing power fall =>
income effect: quantity of potatoes tăng; substitution effect:
quantity of potatoes giảm
○​ If income effect > substitution effect => potatoes are a Giffen
good, a good for which an increase in price raises the quantity
demanded.

●​ Application 2: Wages and labor supply