What is Consumer Equilibrium, Demand Curve & Exercises in Class 12 Economics NCERT?

Interactive NCERT Class 12 Economics lesson on Consumer Equilibrium, Demand Curve & Exercises

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Consumer Equilibrium, Demand Curve & Exercises

🎓 Class 12 Economics CBSE Theory Chapter 2 — Theory of Consumer Behaviour ⏱ ~28 min

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Class 12 · Introductory Microeconomics · Chapter 2 · Part 3

Consumer's Equilibrium, Derivation of the Demand Curve & Exercises

Part 1 gave us cardinal utility. Part 2 set up the indifference map and the budget line. Now we put the two together. Where exactly does the consumer settle? At the magical point where the budget line just kisses the highest reachable indifference curve — the tangency. From this single condition the textbook derives an entire demand curve, decomposes a price change into substitution and income effects, and proves the Law of Demand. This part closes with full model answers to every NCERT exercise question of the chapter, plus a tight summary and key-term glossary for revision.

2.8 Consumer's Equilibrium under the Ordinal Approach

The consumer is rational. From the bundles she can afford (her budget set), she picks the one that gives her the highest satisfaction (the highest reachable indifference curve). The diagrams of Part 2 force three conclusions about where her optimum can be:

⬜

Not below the budget line

A point strictly below the budget line wastes income — by monotonicity, there is always a point on the line that has more of at least one good and is therefore preferred.

⛔

Not above the budget line

Bundles above the budget line are simply unaffordable. The consumer cannot reach them with her current income at the prevailing prices.

📍

On the budget line, at the tangency

Of all points on the budget line, only the one where the line just touches an IC sits on the highest reachable curve. Any other point on the line lies on a lower IC — and is therefore inferior.

📖 Equilibrium Condition

The optimum bundle is located where the budget line is tangent to an indifference curve. At the tangency, the slope of the IC and the slope of the budget line are equal in absolute value. That gives:

At equilibrium: MRS = p₁ ÷ p₂
Second-order condition: the IC must be convex to the origin

2.8.1 Why MRS = Price Ratio (Verbal Argument)

Imagine MRS = 2 and the price ratio = 1 (both goods cost the same). MRS = 2 means the consumer is willing to give up 2 mangoes for one extra banana. Price ratio = 1 means the market only requires her to give up 1 mango. Clearly she gains by buying the extra banana — and she keeps gaining until MRS falls (LDMU + diminishing MRS) to match the price ratio. A symmetric argument applies if MRS < price ratio: she gains by buying less of good 1. So at the optimum the two rates must be equal.

Figure 2.12 — Consumer's equilibrium at E. The budget line is tangent to IC*. At the tangency, slope of IC = slope of budget line ⇒ MRS = p₁/p₂. The dotted lower IC is reachable but inferior; the dotted upper IC is preferred but unaffordable.

⚙️ Two Conditions for Equilibrium

First-order: MRS = p₁/p₂ — the slope of IC equals the slope of the budget line.
Second-order: The IC must be convex to the origin at the tangency. Without convexity (e.g. if MRS were rising), the tangent point could be a worst point on the line, not the best.

2.9 Effects of Changes — Income, Price & the Two Famous Curves

The optimum E shifts every time something the consumer faces changes. Two natural questions follow: (a) what happens when income changes, prices fixed; (b) what happens when one price changes, income fixed.

2.9.1 Income Effect & the Income Consumption Curve (ICC)

Hold prices constant and let income rise from M to M′ to M″. The budget line shifts parallel outward at each step. Each new budget line is tangent to a fresh, higher indifference curve, giving a new optimum E, E′, E″. Joining these consumer-equilibrium points traces the Income Consumption Curve (ICC)^?. The ICC summarises how the consumer's basket evolves with income.

If both goods are normal, the ICC slopes upward — quantities of both goods rise with income.
If one good is inferior, the ICC bends backward at higher income — that good's quantity falls as income rises.

2.9.2 Price Effect, Substitution Effect & Income Effect

Hold income and the price of mangoes constant, and let only p₁ (the price of bananas) fall. The budget line pivots outward around the vertical intercept and becomes flatter (Part 2). The consumer reaches a new tangency point on a higher IC. The total movement of the optimum bundle in response to the price change is the price effect. It can be neatly broken into two pieces:

↔️

Substitution Effect

Bananas became relatively cheaper than mangoes. Even at the original satisfaction level, the consumer would now substitute bananas for mangoes — the slope-of-budget-line argument forces this.

💸

Income Effect

A fall in p₁ raises the consumer's real purchasing power (the same income now buys more). This pushes the consumer onto a higher IC — usually raising consumption of both goods (if normal).

Price Effect = Substitution Effect + Income Effect (the Slutsky / Hicks decomposition)

For most goods both effects pull in the same direction (cheaper good → buy more), so the demand curve slopes downward. For an inferior good, the income effect pulls the other way; if it is strong enough, the demand curve can even slope upward — the famous Giffen good case discussed in Section 2.4.3 of NCERT.

2.9.3 Price Consumption Curve (PCC)

The locus of consumer equilibria as price of one good (say bananas) varies, with income and the other price held fixed, is called the Price Consumption Curve (PCC)^?. Each point on the PCC is a tangency of an outward-pivoting budget line with a higher IC. The PCC is the bridge between the indifference-curve world and the demand curve.

2.10 Deriving the Demand Curve from the PCC

Pick three different prices for bananas: p′₁ > p₁ > p″₁. Each yields a distinct budget line and a distinct optimum on a distinct IC.

At price p′₁ (highest), the consumer's optimum is C, with X′₁ bananas. Plot (p′₁, X′₁) on a price-quantity graph.
At price p₁ (middle), the optimum is D, with X₁ > X′₁ bananas. Plot (p₁, X₁).
At price p″₁ (lowest), the optimum is E, with X″₁ > X₁ bananas. Plot (p″₁, X″₁).

Join the three (price, quantity) points and you have the consumer's demand curve. Because each fall in price yields a higher quantity, the curve is negatively sloped. This is precisely the Law of Demand.

Figure 2.14 — From PCC to demand curve. Each fall in p₁ pivots the budget line outward, reaches a new tangency on a higher IC, and adds one (price, quantity) point to the demand schedule. The negative slope of D is the Law of Demand.

2.10.1 The Law of Demand — Stated Formally

📖 Law of Demand — NCERT

Other things being equal, there is a negative relationship between the demand for a commodity and its price. When the price rises, the quantity demanded falls; when the price falls, the quantity demanded rises.

Two complementary explanations of why the law holds:

Cardinal explanation (Part 1): the LDMU means each extra unit gives less MU, so the consumer pays less for additional units → demand curve slopes down.
Ordinal explanation (this part): a price fall pivots the budget line outward; the new tangency lies further along the X-axis. Substitution and income effects both raise demand for a normal good.

2.11 Summary of the Chapter

📌 Chapter 2 — At a Glance (NCERT)

The budget set is the collection of all bundles a consumer can buy with her income at the prevailing market prices.
The budget line contains all bundles that exhaust the entire income; it is downward sloping with slope − p₁/p₂.
The budget set changes when any of the two prices or income changes.
The consumer has well-defined preferences over bundles and can rank them.
Preferences are assumed monotonic: more of any good is better, other things equal.
An indifference curve is the locus of bundles among which the consumer is indifferent. Monotonicity makes ICs slope downward.
An indifference map is the family of all ICs; preferences can equivalently be represented by a utility function.
A rational consumer chooses her most preferred bundle from the budget set. The optimum sits at the tangency between the budget line and an IC.
The demand curve records the quantity she chooses at each price, holding other things fixed; it is generally downward sloping.
Demand for a normal good rises with income; demand for an inferior good falls with income.
The market demand curve is the horizontal sum of all individual demand curves.
Price elasticity of demand = (% change in quantity demanded) ÷ (% change in price); a pure number, related to total expenditure.

2.12 Complete NCERT Exercises — Worked Solutions

The following exercises are reproduced from NCERT Class 12 Introductory Microeconomics, Chapter 2. Each question is followed by a model answer.

What do you mean by the budget set of a consumer?

The budget set of a consumer is the collection of all bundles of two goods that she can afford to buy with her given income at the prevailing market prices. Formally, with income M and prices p₁ and p₂, the budget set contains every bundle (x₁, x₂) such that p₁x₁ + p₂x₂ ≤ M.

What is budget line?

The budget line is the boundary of the budget set. It is the graph of the equation p₁x₁ + p₂x₂ = M, showing all bundles that cost the consumer her entire income. Bundles below the line are affordable but leave income unspent; bundles above the line are unaffordable.

Explain why the budget line is downward sloping.

To stay on the budget line, the consumer must spend exactly M. If she wants more of good 1, she must spend more on good 1, and therefore must spend less on good 2. Less spending on good 2 means a smaller quantity of good 2. So an increase in x₁ forces a decrease in x₂ — a negative slope. Algebraically, slope = − p₁/p₂, which is negative because both prices are positive.

A consumer wants to consume two goods. The prices of the two goods are ₹4 and ₹5 respectively. The consumer's income is ₹20.
(i) Write down the equation of the budget line. (ii) How much of good 1 if she spends entire income on it? (iii) How much of good 2 if she spends entire income on it? (iv) What is the slope of the budget line?

With p₁ = 4, p₂ = 5, M = 20:
(i) Equation: 4x₁ + 5x₂ = 20.
(ii) If she buys only good 1: x₁ = M/p₁ = 20/4 = 5 units.
(iii) If she buys only good 2: x₂ = M/p₂ = 20/5 = 4 units.
(iv) Slope = − p₁/p₂ = − 4/5 (i.e. − 0.8).

How does the budget line change if the consumer's income increases to ₹40 but the prices remain unchanged?

New equation: 4x₁ + 5x₂ = 40. New intercepts: x₁ max = 40/4 = 10, x₂ max = 40/5 = 8 (both doubled). Slope is unchanged at − 4/5 because prices are unchanged. The budget line shifts parallel outward — the consumer can now afford bundles with twice as much of either good (or any combination in between).

How does the budget line change if the price of good 2 decreases by a rupee but the price of good 1 and the consumer's income remain unchanged?

New p₂ = 4. New equation: 4x₁ + 4x₂ = 20. Intercepts: x₁ max = 20/4 = 5 (unchanged); x₂ max = 20/4 = 5 (rises from 4 to 5). New slope = − 4/4 = − 1 (steeper). The budget line pivots outward around the horizontal intercept (5, 0) — the same as the NCERT case where p₁ falls, applied to good 2. The consumer can now afford more good 2 at every level of good 1.

What happens to the budget set if both the prices as well as the income double?

New equation: (2p₁)x₁ + (2p₂)x₂ = 2M. Dividing both sides by 2 gives p₁x₁ + p₂x₂ = M — the identical equation as before. So the budget line and budget set are completely unchanged. Doubling all nominal magnitudes is a "money illusion" that leaves the real purchasing power identical.

Suppose a consumer can afford to buy 6 units of good 1 and 8 units of good 2 if she spends her entire income. The prices of the two goods are ₹6 and ₹8 respectively. How much is the consumer's income?

The bundle (6, 8) lies on the budget line, so M = p₁x₁ + p₂x₂ = 6 × 6 + 8 × 8 = 36 + 64 = ₹100.

A consumer wants to consume two goods which are available only in integer units. The two goods are equally priced at ₹10 and the consumer's income is ₹40.
(i) Write down all the bundles that are available. (ii) Among the available bundles, identify those which cost exactly ₹40.

With p₁ = p₂ = 10, M = 40, condition is 10x₁ + 10x₂ ≤ 40, i.e. x₁ + x₂ ≤ 4 with x₁, x₂ ∈ {0,1,2,3,4}.
(i) Affordable bundles: (0,0), (0,1), (0,2), (0,3), (0,4), (1,0), (1,1), (1,2), (1,3), (2,0), (2,1), (2,2), (3,0), (3,1), (4,0).
(ii) Bundles costing exactly ₹40: (0,4), (1,3), (2,2), (3,1), (4,0) — they satisfy x₁ + x₂ = 4 and lie on the budget line.

Q10

What do you mean by 'monotonic preferences'?

Preferences are monotonic if, between any two bundles, the consumer prefers the one that has more of at least one good and no less of the other good. Loosely: "more is always better" so long as you don't have less of anything else. Monotonicity rules out situations where a clearly larger bundle leaves the consumer indifferent.

Q11

If a consumer has monotonic preferences, can she be indifferent between the bundles (10, 8) and (8, 6)?

Bundle (10, 8) has more of good 1 (10 vs 8) and more of good 2 (8 vs 6) compared with (8, 6). By monotonicity she must strictly prefer (10, 8) to (8, 6). Therefore she cannot be indifferent between them.

Q12

Suppose a consumer's preferences are monotonic. What can you say about her preference ranking over the bundles (10, 10), (10, 9) and (9, 9)?

Compare (10,10) with (10,9): same good 1, more good 2 in the first → (10,10) is preferred to (10,9). Compare (10,9) with (9,9): same good 2, more good 1 in (10,9) → (10,9) is preferred to (9,9). By transitivity, (10,10) is preferred to (9,9). Final ranking: (10, 10) ≻ (10, 9) ≻ (9, 9).

Q13

Suppose your friend is indifferent to the bundles (5, 6) and (6, 6). Are the preferences of your friend monotonic?

Bundle (6, 6) has more of good 1 (6 vs 5) and the same of good 2 compared with (5, 6). Monotonicity demands that (6, 6) be strictly preferred to (5, 6). The friend says she is indifferent — which contradicts monotonicity. So her preferences are not monotonic.

Q14

Suppose there are two consumers in the market for a good and their demand functions are:
d₁(p) = 20 − p for p ≤ 20, and d₁(p) = 0 at p > 20.
d₂(p) = 30 − 2p for p ≤ 15 and d₂(p) = 0 at p > 15.
Find out the market demand function.

Market demand = d₁(p) + d₂(p) where each term is the relevant individual function in its valid range.
• For 0 ≤ p ≤ 15: both consumers active. D(p) = (20 − p) + (30 − 2p) = 50 − 3p.
• For 15 < p ≤ 20: consumer 2 demands 0; consumer 1 still active. D(p) = 20 − p.
• For p > 20: both consumers demand 0. D(p) = 0.

Q15

Suppose there are 20 consumers for a good and they have identical demand functions:
d(p) = 10 − 3p for p ≤ 10/3, and d(p) = 0 at p > 10/3.
What is the market demand function?

Identical consumers ⇒ market demand = 20 × individual demand.
• For 0 ≤ p ≤ 10/3: D(p) = 20 × (10 − 3p) = 200 − 60p.
• For p > 10/3: D(p) = 0.

Q16

Consider a market where there are just two consumers and suppose their demands for the good are given as follows:
p = 1, d₁ = 9, d₂ = 24
p = 2, d₁ = 8, d₂ = 20
p = 3, d₁ = 7, d₂ = 18
p = 4, d₁ = 6, d₂ = 16
p = 5, d₁ = 5, d₂ = 14
p = 6, d₁ = 4, d₂ = 12
Calculate the market demand for the good.

Market demand at each price = d₁ + d₂.
p=1 → 9 + 24 = 33.
p=2 → 8 + 20 = 28.
p=3 → 7 + 18 = 25.
p=4 → 6 + 16 = 22.
p=5 → 5 + 14 = 19.
p=6 → 4 + 12 = 16.
Market demand falls as price rises — the Law of Demand at the market level.

Q17

What do you mean by a normal good?

A normal good is a good whose demand increases with the consumer's income and falls with a fall in income, all else constant. Demand and income move in the same direction. Most everyday goods (clothing, restaurant meals, electricity for households) are normal goods.

Q18

What do you mean by an 'inferior good'? Give some examples.

An inferior good is one whose demand falls when the consumer's income rises, and rises when income falls. Demand and income move in opposite directions. Typical examples: low-quality cereals (coarse rice, bajra), second-hand clothing, public-bus journeys (when richer people switch to taxis or own cars), and basic instant noodles in some contexts.

Q19

What do you mean by substitutes? Give examples of two goods which are substitutes of each other.

Substitute goods are goods that satisfy the same need; one can replace the other in consumption. The demand for a good moves in the same direction as the price of its substitute (price of coffee ↑ → people switch to tea → demand for tea ↑). Examples: tea and coffee, Pepsi and Coca-Cola, train travel and bus travel.

Q20

What do you mean by complements? Give examples of two goods which are complements of each other.

Complementary goods are consumed together; one is incomplete without the other. The demand for a good moves in the opposite direction of the price of its complement (price of sugar ↑ → tea becomes more expensive in total → demand for tea ↓). Examples: tea and sugar, shoes and socks, pen and ink, car and petrol.

Q21

Explain price elasticity of demand.

Price elasticity of demand (e_D) measures the responsiveness of the quantity demanded of a good to a change in its price. Formally:

e_D = (% change in quantity demanded) ÷ (% change in price) = (ΔQ/Q) × (P/ΔP)

Although strictly negative (inverse relation), we usually quote its absolute value. If |e_D| > 1, demand is elastic; if |e_D| < 1, demand is inelastic; if |e_D| = 1, demand is unitary elastic. Necessities (food, salt) tend to be inelastic; luxuries and goods with close substitutes tend to be elastic.

Q22

Consider the demand for a good. At price ₹4, the demand is 25 units. Suppose price rises to ₹5, and as a result demand falls to 20 units. Calculate the price elasticity.

Q₁ = 25, Q₂ = 20 ⇒ ΔQ = −5. P₁ = 4, P₂ = 5 ⇒ ΔP = +1.
% change in Q = (−5 / 25) × 100 = −20%.
% change in P = (1 / 4) × 100 = +25%.
e_D = (−20%) ÷ (25%) = − 0.8; absolute value = 0.8 < 1, so demand is price inelastic.

Q23

Consider the demand curve D(p) = 10 − 3p. What is the elasticity at price 5/3?

For a linear demand curve q = a − bp, the slope dq/dp = −b. Here b = 3.
At p = 5/3: q = 10 − 3 × (5/3) = 10 − 5 = 5.
e_D = (dq/dp) × (p/q) = (−3) × ((5/3) / 5) = (−3) × (1/3) = −1.
Absolute value = 1, so the demand at this price is unit elastic — exactly the midpoint of this linear demand curve.

Q24

Suppose the price elasticity of demand for a good is − 0.2. If there is a 5% increase in the price of the good, by what percentage will the demand for the good go down?

By definition, e_D = (% ΔQ) / (% ΔP). Rearranging: % ΔQ = e_D × % ΔP = (−0.2) × (+5%) = −1%. Demand falls by 1%.

Q25

Suppose the price elasticity of demand for a good is − 0.2. How will the expenditure on the good be affected if there is a 10% increase in the price of the good?

% ΔQ = e_D × % ΔP = (−0.2) × (10%) = −2%. Expenditure E = P × Q. Approximate % ΔE ≈ % ΔP + % ΔQ = (+10%) + (−2%) = +8%. Since |e_D| < 1, demand is inelastic and expenditure moves in the same direction as price — exactly as Table 2.5 of NCERT predicts. Expenditure on the good rises by about 8%.

Q26

Suppose there was a 4% decrease in the price of a good, and as a result, the expenditure on the good increased by 2%. What can you say about the elasticity of demand?

Approximate relationship: % ΔE ≈ % ΔP + % ΔQ. Plug in: 2 = (−4) + % ΔQ ⇒ % ΔQ = +6%.
e_D = (% ΔQ) / (% ΔP) = (+6) / (−4) = − 1.5; absolute value = 1.5 > 1, so demand is elastic. (When price falls and expenditure rises, the good must be price elastic — quantity rises proportionately more than price falls.)

📝 Competency-Based Questions — Apply, Analyse, Evaluate, Create

Scenario. Anya has ₹100 to spend on books (P_B = ₹10) and movie tickets (P_M = ₹20). At her current bundle (4 books, 3 tickets), her MRS of tickets-for-books is 2. The price of movie tickets then falls to ₹10.

Q1. Verify whether Anya's current bundle (4, 3) is on her budget line and whether she is in equilibrium. If not, advise her on which good to buy more of.

L3 Apply

Answer. Cost of (4, 3) = 10 × 4 + 20 × 3 = 40 + 60 = ₹100 — exactly the budget. So she is on the budget line. Price ratio = P_B / P_M = 10 / 20 = 0.5. Her MRS = 2 (a movie ticket gives her satisfaction equal to 2 books). Since MRS > price ratio, the willingness to substitute exceeds the market rate — she is not in equilibrium. She should buy more books and fewer tickets; as she does, MRS falls (diminishing MRS) until MRS = 0.5 = price ratio.

Q2. After P_M falls from ₹20 to ₹10, draw out (verbally) what happens to Anya's budget line and her optimum bundle, decomposing the change into substitution and income effects.

L4 Analyse

Answer. New budget line: 10B + 10M = 100. Vertical intercept (tickets only) rises from 5 → 10. Horizontal intercept (books only) unchanged at 10. The line pivots outward around the books axis — flatter slope (now − 0.5 instead of − 1 in the (Tickets, Books) plot, or equivalently the price ratio P_M / P_B drops from 2 to 1). Substitution effect: even at original satisfaction, tickets are now cheaper relative to books, so Anya buys more tickets, fewer books. Income effect: lower P_M raises her real income; if both goods are normal, she also buys more of both. Total price effect = both forces add up → tickets rise unambiguously, books may rise or fall depending on the relative sizes of the two effects.

Q3. Suppose Anya's friend Reema has identical preferences but P_M = ₹40 in her city. Draw a comparative reasoning for the slope of the budget line and the position of equilibrium.

L4 Analyse

Answer. Reema's budget line: 10B + 40M = 100, slope = − 10/40 = − 0.25 (vs Anya's − 0.5). Reema's line is much flatter (in B vs M space, with M on the X-axis). At equilibrium, MRS = price ratio = 0.25 — so Reema's optimum is reached at a much lower MRS than Anya's. Diminishing MRS implies Reema must consume more books and fewer tickets than Anya at the optimum, even though they share preferences. Higher movie-ticket prices simply re-route the same consumer toward the relatively cheaper good.

Q4. The Government places a 50% tax on books, raising P_B from ₹10 to ₹15 (P_M still ₹20, M still ₹100). Predict and justify the qualitative direction of Anya's new optimum bundle and explain the role of the substitution and income effects.

L5 Evaluate

Answer. Books are now relatively more expensive (price ratio P_B/P_M rises from 0.5 to 0.75). Substitution effect: Anya substitutes movies for books — buys fewer books, more tickets. Income effect: real purchasing power falls because one good is dearer; if both are normal, Anya cuts back on both. Net effect on books: substitution (fewer books) and income (fewer books) both point downward → books fall unambiguously. Net effect on tickets: substitution (more tickets) versus income (fewer tickets) — the direction depends on relative strengths but typically tickets rise for a normal good with a moderate income effect. The tax thus achieves both a price effect on books and a smaller real-income squeeze.

HOT Q5. A student claims, "Indifference curves and budget lines together don't predict the demand curve — they just describe one consumer's optimum." Argue, in two short paragraphs, why the demand curve is in fact a direct logical consequence of the IC + budget-line apparatus.

L6 Create

Answer. Paragraph 1. Fix income, the price of good 2, and the consumer's preferences. Vary p₁ continuously. Each value of p₁ gives one budget line, which is tangent to one IC at one point — the consumer's optimum. The X-coordinate of that optimum is the quantity of good 1 the consumer chooses at that price. Map every (p₁, x₁*) pair onto a price-quantity diagram and you have just constructed a demand curve. Nothing more is needed. Paragraph 2. Moreover, the IC + budget-line framework predicts the direction of the curve, not just its existence: a fall in p₁ pivots the budget line outward, the new tangency lies further along the X-axis (substitution + income effects for a normal good), so x₁* must rise. That is exactly the negative slope of the demand curve — the Law of Demand. The student is therefore wrong: the demand curve is a direct, predictable consequence of the apparatus, not an extra assumption.

🎯 Assertion–Reason Questions

Assertion (A): The consumer's optimum bundle lies at the point where the budget line is tangent to an indifference curve.
Reason (R): At the tangency, the slope of the indifference curve (MRS) equals the slope of the budget line (price ratio).

Options: (a) Both A and R are true and R is the correct explanation of A. (b) Both A and R are true but R is not the correct explanation of A. (c) A is true, R is false. (d) A is false, R is true.

Correct answer: (a) — The tangency is exactly where the rate at which the consumer is willing to substitute (MRS) equals the rate at which the market lets her substitute (price ratio); only then is no further reshuffling profitable. Therefore R is the cause and A the consequence.

Assertion (A): A fall in the price of a normal good unambiguously raises the quantity demanded.
Reason (R): For a normal good the substitution effect and income effect of a price fall both push consumption upward.

Correct answer: (a) — Substitution effect: cheaper good replaces dearer ones (↑). Income effect for a normal good: higher real income raises consumption of normal goods (↑). Both arrows point up, so quantity demanded must rise. R is precisely the reason for A.

Assertion (A): The demand curve derived from the indifference-curve framework is generally negatively sloped.
Reason (R): The Price Consumption Curve (PCC) traces the consumer's optimum as the price of one good varies, and each lower price is associated with a higher quantity demanded for a normal good.

Correct answer: (a) — The demand curve is the (price, quantity) projection of the PCC. Since lower prices yield higher quantities (as long as both effects point the same way for normal goods), the projection slopes downward. R is the geometric mechanism behind A.

LET'S EXPLORE — Construct a Demand Schedule from a PCC

A consumer has ₹120, p₂ = ₹10. Let p₁ take values ₹4, ₹6, ₹10, ₹15, ₹20.
For each p₁, find the horizontal intercept of the budget line (M / p₁). Pretend the consumer always chooses 60% of that maximum (a behavioural rule). That gives x₁* at each price.
Tabulate (p₁, x₁*) and plot the demand curve. Verify it slopes downward.
Now imagine income rises to ₹180. Repeat the exercise. Does the demand curve shift right?

Sample finding: Horizontal intercepts at M=120: 30, 20, 12, 8, 6. 60% of each: 18, 12, 7.2, 4.8, 3.6. So at p=4 → q=18; p=6 → q=12; p=10 → q=7.2; p=15 → q=4.8; p=20 → q=3.6. Plot — clearly downward sloping. With M=180, intercepts become 45, 30, 18, 12, 9; 60% = 27, 18, 10.8, 7.2, 5.4 — every quantity rises by 50%, demand curve shifts right (a normal good).

2.13 Key Concepts (NCERT)

Budget Set

All bundles affordable at the prevailing prices given the consumer's income.

Budget Line

Bundles that exhaust the entire income; equation p₁x₁ + p₂x₂ = M.

Preference

Consumer's ordering of bundles — prefers, indifferent, or less preferred.

Indifference Curve

Locus of bundles giving the same satisfaction.

Marginal Rate of Substitution

|ΔY/ΔX| along an IC; absolute slope of the IC.

Diminishing Rate of Substitution

MRS falls as the consumer obtains more of one good — gives ICs their convex shape.

Monotonic Preferences

More of any good is better, holding the others constant.

Indifference Map

Family of indifference curves; higher curves represent higher utility.

Utility Function

A numerical representation of the consumer's preferences over bundles.

Consumer's Optimum

Bundle on the highest reachable IC tangent to the budget line; MRS = p₁/p₂.

Demand

Quantity a consumer is willing and able to buy at a given price.

Demand Curve

Graph of quantity demanded against price; generally downward sloping.

Law of Demand

Quantity demanded varies inversely with price, other things equal.

Substitution Effect

Change in quantity demanded due to a change in relative prices, real income held constant.

Income Effect

Change in quantity demanded due to a change in real income, prices held constant.

Price Consumption Curve

Locus of consumer optima as the price of one good varies; basis for the demand curve.

Income Consumption Curve

Locus of consumer optima as income varies; shows how the basket evolves with income.

Normal Good

Demand rises with income, falls with falling income.

Inferior Good

Demand falls with rising income, rises with falling income.

Substitute

A good that can replace another in consumption; demand and substitute's price move together.

Complement

A good consumed jointly with another; demand and complement's price move oppositely.

Price Elasticity of Demand

% change in Q ÷ % change in P; pure number; measures responsiveness.

Frequently Asked Questions — Consumer's Equilibrium, Derivation of the Demand Curve & Exercises

What is the consumer's equilibrium condition under the ordinal approach?

Under the ordinal approach, the consumer is in equilibrium at the point where the highest attainable indifference curve is tangent to the budget line. At this tangency the slope of the indifference curve equals the slope of the budget line, giving MRS = p1/p2. The consumer cannot reach a higher curve while staying within the budget set, so this bundle maximises utility.

How is the demand curve derived from the price consumption curve?

Starting from a consumer's equilibrium, lower the price of good 1 while holding income and the price of good 2 constant. The budget line rotates outward along the x1 axis, and a new tangency gives a new equilibrium with more of good 1. Repeating this for many prices gives the price consumption curve (PCC). Reading off price and quantity from each equilibrium and plotting them produces the downward-sloping demand curve for good 1.

What is the price consumption curve (PCC)?

The price consumption curve traces the consumer's optimal bundles as the price of one good changes while income and the price of the other good are held constant. Each point on the PCC is a tangency between an indifference curve and a budget line at the new price. The PCC underlies the derivation of the individual demand curve in the ordinal approach.

What is the income consumption curve (ICC)?

The income consumption curve traces the consumer's optimal bundles as money income changes while both prices are held fixed. As income rises, the budget line shifts outward in parallel and a new tangency is found. For a normal good both axes show more consumption as income rises, so ICC slopes upward. For an inferior good the quantity falls as income rises beyond a level, bending the ICC backward.

What is the difference between a normal good and an inferior good?

A normal good is one whose demand rises when consumer income rises, holding prices constant — most goods like clothing and consumer durables. An inferior good is one whose demand falls when income rises because consumers substitute towards superior alternatives — coarse cereals and low-quality clothing in NCERT examples. The income elasticity of demand is positive for normal goods and negative for inferior goods.

Why does the demand curve slope downward?

The demand curve slopes downward because of the substitution effect and the income effect. When the price of a good falls the good becomes cheaper relative to others, so the consumer substitutes towards it (substitution effect). The lower price also raises real income, so the consumer can buy more (income effect). For a normal good both effects work in the same direction, producing the standard inverse price-quantity relationship.

What are some sample NCERT Class 12 Chapter 2 exercise topics?

NCERT Class 12 Chapter 2 exercises ask students to define utility, draw indifference curves with given properties, derive consumer equilibrium given prices and income, analyse the budget line under price and income changes, distinguish monotonic and convex preferences, and derive the demand curve from PCC. All exercises are worked through in this part with full model answers.

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