CUET UG Economics Microeconomics Consumer Theory Utility Indifference Curves Budget Constraint

Must-Know

• Utility is the satisfaction derived from consuming a good; total utility (TU) increases at a diminishing rate as more units are consumed. Example: 1st ice cream gives 20 utils, 2nd gives 15, total becomes 35.
• Marginal utility (MU) = Change in TU / Change in quantity. If TU increases from 35 to 45 when consumption rises from 2 to 3 units, MU = (45–35)/(3–2) = 10.
• Law of Diminishing Marginal Utility: As consumption increases, MU from each additional unit falls, assuming continuous consumption and rational behavior.
• Consumer equilibrium in cardinal utility theory occurs when MUₓ/Pₓ = MUᵧ/Pᵧ = MUm (marginal utility of money), with all income spent.
• If MUₓ/Pₓ > MUᵧ/Pᵧ, consumer increases consumption of good X and reduces Y to maximize utility.
• Indifference curve (IC) shows combinations of two goods giving same satisfaction; downward sloping due to trade-off between goods.
• Higher IC represents higher utility; ICs never intersect because of transitivity and consistency in preferences.
• IC is convex to origin due to diminishing marginal rate of substitution (MRS). MRS = ΔY/ΔX = MUₓ/MUᵧ.
• Budget line equation: PₓX + PᵧY = M, where M is income, Pₓ and Pᵧ are prices. Example: If Pₓ = ₹10, Pᵧ = ₹5, M = ₹100 → 10X + 5Y = 100.
• Slope of budget line = –Pₓ/Pᵧ; represents opportunity cost of good X in terms of good Y.
• Consumer equilibrium under ordinal utility: IC tangent to budget line, where MRS = Pₓ/Pᵧ.
• At equilibrium, the rate at which consumer is willing to substitute (MRS) equals rate at which market allows (price ratio).
• If MRS > Pₓ/Pᵧ, consumer buys more of X; if MRS < Pₓ/Pᵧ, sells X (or buys less).
• Increase in income shifts budget line parallel outward (normal goods) or inward (inferior goods); price change rotates budget line.
• Price-consumption curve (PCC) joins equilibrium points when price of one good changes, others constant.
• Income-consumption curve (ICC) joins equilibrium points as income changes, prices constant.
• Perfect substitutes have straight-line ICs (e.g., Coke and Pepsi); MRS constant.
• Perfect complements have L-shaped ICs (e.g., left and right shoes); MRS = 0 or ∞.
• Marginal rate of substitution (MRS) diminishes along an IC due to increasing opportunity cost of giving up one good.
• A Giffen good is a special type of inferior good where demand rises with price due to strong income effect outweighing substitution effect. Example: verified from NCERT (potato in 19th century Ireland – mentioned as illustration).

Difficulty Level

Intermediate — combines abstract concepts (utility, indifference curves) with graphical analysis and numerical application, requiring conceptual clarity and diagram interpretation common in CUET.

Common CUET Traps

• Trap: Assuming that higher indifference curves can intersect. Avoid: ICs never intersect due to logical inconsistency in preferences; if they did, same bundle would give two different satisfaction levels.
• Trap: Confusing slope of budget line with MRS. Avoid: Slope of budget line is –Pₓ/Pᵧ (market rate), while MRS is –ΔY/ΔX (consumer’s willingness); equal only at equilibrium.
• Trap: Thinking consumer equilibrium requires MUₓ = MUᵧ. Avoid: Equilibrium requires MUₓ/Pₓ = MUᵧ/Pᵧ, not equality of marginal utilities; depends on prices.

Practice MCQs

1. Question: What happens to marginal utility when total utility is maximum?
   A) MU is positive
   B) MU is zero
   C) MU is negative
   D) MU is equal to total utility
   Answer: B
   Explanation: At the point of maximum TU, MU becomes zero (saturation point).
   Why others fail: Option A is true before maximum TU, but not at peak.

2. Question: Which of the following defines the slope of the budget line?
   A) –MUₓ/MUᵧ
   B) –Pᵧ/Pₓ
   C) –Pₓ/Pᵧ
   D) –ΔX/ΔY
   Answer: C
   Explanation: Slope of budget line is negative ratio of prices: –Pₓ/Pᵧ.
   Why others fail: Option A is MRS, often confused with price ratio.

3. Question: A consumer consumes two goods X and Y. At equilibrium, it must be true that:
   A) MUₓ = MUᵧ
   B) TUₓ = TUᵧ
   C) MUₓ/Pₓ = MUᵧ/Pᵧ
   D) Pₓ = Pᵧ
   Answer: C
   Explanation: Equilibrium condition in cardinal utility theory is equality of MU per rupee spent.
   Why others fail: Option A ignores price differences; students often forget division by price.

4. Question: An indifference curve is convex to the origin because:
   A) Consumers prefer more to less
   B) MRS diminishes
   C) Goods are perfect substitutes
   D) Budget line is linear
   Answer: B
   Explanation: Convexity reflects diminishing MRS – consumer gives up less of Y for each additional unit of X.
   Why others fail: Option A explains downward slope, not convex shape.

5. Question: Suppose the price of good X falls, and as a result, the quantity demanded also falls. This could describe:
   A) Normal good
   B) Inferior good
   C) Giffen good
   D) Substitute good
   Answer: C
   Explanation: Giffen goods are inferior goods where income effect dominates, causing demand to fall when price falls.
   Why others fail: Option B (inferior) is partially correct, but only Giffen goods show upward-sloping demand curve.

Last‑Minute Revision

• ⚠️ MU becomes zero when TU is maximum.
• ⚠️ Law of DMU assumes rational consumer, continuous consumption, and cardinal measurement.
• ⚠️ TU increases as long as MU is positive.
• ⚠️ Consumer equilibrium: MUₓ/Pₓ = MUᵧ/Pᵧ = MUm (cardinal), MRS = Pₓ/Pᵧ (ordinal).
• ⚠️ Budget line shifts outward with income increase, rotates with price change.
• ⚠️ MRS = –ΔY/ΔX = MUₓ/MUᵧ (diminishes along IC).
• ⚠️ ICs are downward sloping due to trade-off between goods.
• ⚠️ ICs do not intersect – violates transitivity.
• ⚠️ Higher IC → higher satisfaction level.
• ⚠️ Convex IC → diminishing MRS.
• ⚠️ Straight-line IC → perfect substitutes (MRS constant).
• ⚠️ L-shaped IC → perfect complements (e.g., shoes).
• ⚠️ Giffen good: demand ↑ when price ↑; inferior with strong income effect.
• ⚠️ Substitution effect always negative (price ↑ → quantity ↓).
• ⚠️ Income effect for inferior good is positive (income ↑ → demand ↓).
• ⚠️ PCC traces equilibrium as price changes; ICC as income changes.
• ⚠️ Slope of budget line = –Pₓ/Pᵧ.
• ⚠️ At equilibrium, IC is tangent to budget line.
• ⚠️ MU of money is assumed constant in cardinal utility analysis.
• ⚠️ Verify from NCERT: Example of Giffen good – low-income staple food (e.g., potatoes).