A rise in consumer income raises the quantity of a normal good bought at every price. This is correctly described as:

A movement along the demand curve

Two indifference curves of the same consumer cannot intersect because intersection would violate:

The transitivity of preferences

The law of variable proportions applies to a situation in which:

At least one factor is fixed and a variable factor is added to it

All factors are increased in the same proportion

All factors are fixed and only technology changes

Output is fixed and the factor mix is chosen to minimise cost

Under perfect competition the marginal revenue curve of an individual firm:

Coincides with its average revenue curve and is horizontal

Lies below its average revenue curve and slopes downward

Lies above its average revenue curve

Is vertical at the equilibrium output

Baumol's model of the firm holds that managers maximise:

Sales revenue, subject to a minimum profit constraint

Staff and discretionary expenditure

On a straight-line demand curve, the price elasticity of demand at the midpoint is:

The same as at every other point on the line

Business Economics I: Demand, Consumer Behaviour and Production

Unit 3 of the NET Commerce syllabus is microeconomics put to work inside a firm. It opens with the scope of business economics and the objectives of the firm, then runs demand and its elasticity, the AR–MR relationship, utility and indifference curve analysis, and the two laws of production. This chapter covers that first half. Almost every question here is either a definition applied to a case or a small calculation, and the calculations are the ones worth drilling because they are where an otherwise prepared candidate loses marks to a sign or a denominator.

1. Scope of business economics, and the objectives of the firm

Business economics — managerial economics is the same subject under another name — applies economic theory and the tools of decision science to the choices a manager actually faces: what to produce, how much, at what price, and whether to invest. It is microeconomic in its core, normative in its use, and it borrows from statistics and operations research for method.

On objectives, the traditional assumption is profit maximisation, which the syllabus expects you to be able to set against the alternatives that were proposed because real firms are not run by their owners.

Objectives of the firm
Objective	Associated with	Central claim
Profit maximisation	Neoclassical theory	Output is set where marginal revenue equals marginal cost
Sales revenue maximisation	Baumol	Managers maximise sales subject to a minimum profit constraint
Managerial utility maximisation	Williamson	Managers pursue staff, perquisites and discretionary spending
Satisficing	Simon	Bounded rationality makes firms seek an acceptable, not an optimal, result

🎯 Why the alternatives exist at all

They exist because ownership separated from control. A salaried manager's rewards track the size and comfort of the enterprise more closely than its profit, so the objective the firm actually pursues need not be the owners'. Every one of these models is a way of writing that divergence down.

2. Demand, and the exceptions that are not exceptions

The law of demand states that, other things remaining equal, the quantity demanded of a good varies inversely with its price. The clause carrying all the weight is 'other things remaining equal'. A change in the good's own price moves the buyer along a fixed demand curve; a change in anything else — income, the price of a related good, tastes, expectations, the number of buyers — shifts the whole curve.

⚠️ Movement along, versus shift of

'Extension and contraction of demand' are movements along the curve caused by the good's own price. 'Increase and decrease in demand' are shifts of the curve caused by everything else. Questions use the four words precisely and mark on that precision, so a rise in consumer income causes an increase in demand and never an extension of it.

The apparent exceptions are worth stating carefully, because two of the three are not exceptions at all once the ceteris paribus clause is honoured.

Giffen goods — a genuine exception. For a strongly inferior staple taking a large share of a poor household's budget, a price rise so impoverishes the household that it buys more of the staple and less of everything better. The income effect outweighs and reverses the substitution effect.
Veblen or conspicuous goods — the price is itself the attraction, so a price rise raises demand. Strictly this is a change in tastes triggered by price rather than a violation of the law.
Expectation of a further rise — buyers stock up today. Here the relevant price has changed, namely the expected future one, so the ceteris paribus clause has already been broken.

3. Elasticity, measured three ways

Price elasticity of demand is the proportionate change in quantity demanded divided by the proportionate change in price. It is negative for a normal downward-sloping demand curve, and the convention is to report its magnitude.

Percentage or proportionate method — Ed = (ΔQ/Q) ÷ (ΔP/P). Direct, but it gives a different answer depending on whether you start from the old or the new price.
Arc or midpoint method — the same ratio taken over the averages of the two prices and the two quantities, which removes that dependence and is the method to use when the price change is large.
Point method — elasticity on a straight-line demand curve equals the lower segment divided by the upper segment, measured from the point. It follows that elasticity is unity at the midpoint, greater than one above it and less than one below it, and that a single straight line carries every elasticity along its length.

The other two elasticities
Elasticity	Measures	Sign and what it tells you
Income elasticity	Response of quantity demanded to a change in income	Positive for a normal good, negative for an inferior one; above one marks a luxury
Cross elasticity	Response of demand for one good to the price of another	Positive for substitutes, negative for complements, zero for unrelated goods

🧠 Cross elasticity: the sign is the answer

You are almost never asked the magnitude of a cross elasticity, only its sign. Tea and coffee move together in demand when one's price rises, so the sign is positive and they are substitutes. Cars and petrol move apart, so the sign is negative and they are complements. Answer the sign first and read the options afterwards.

4. Average revenue, marginal revenue and elasticity

Average revenue is total revenue divided by output, which for a single-price seller is simply the price. The average revenue curve is therefore the firm's demand curve. Marginal revenue is the addition to total revenue from selling one more unit, and it is below average revenue whenever the seller must cut the price on every unit to sell the extra one.

The relationship the examiner wants is MR = AR × (1 − 1/e), where e is the magnitude of price elasticity. Three readings follow immediately and each has been asked on its own.

When e = 1, MR is zero and total revenue is at its maximum.
When e > 1, MR is positive, so a price cut raises total revenue.
When e < 1, MR is negative, so a price cut lowers total revenue — and a profit-maximising firm therefore never operates where demand is inelastic.
Under perfect competition e is infinite, so MR = AR = price and both curves are the same horizontal line.

5. Utility and indifference curve analysis

Marshall's cardinal utility approach assumes satisfaction can be measured in units. The law of diminishing marginal utility says each successive unit of a good adds less satisfaction than the one before, and the law of equi-marginal utility says a consumer allocates spending so that the marginal utility per rupee is equal across all goods. The consumer is in equilibrium for a single good where marginal utility equals price.

Hicks and Allen's ordinal approach dispenses with measurement and needs only ranking. An indifference curve joins the combinations of two goods among which the consumer is indifferent. Its properties are the examinable part: it slopes downward, is convex to the origin because the marginal rate of substitution diminishes, never intersects another such curve, and represents higher satisfaction the further out it lies. Consumer equilibrium is where the budget line is tangent to the highest attainable indifference curve, so that the marginal rate of substitution equals the price ratio.

🎯 Why two indifference curves cannot cross

If they crossed, the point of intersection would lie on both. It would then be indifferent to a point on the lower curve and to a point on the higher one, which by transitivity would make those two points indifferent to each other — and they cannot be, because one gives more of both goods. The impossibility is a consequence of consistent preferences, not a drawing convention.

6. Variable proportions and returns to scale

The two laws answer different questions and are separated by the time period. The law of variable proportions is a short-run law: at least one factor is fixed, and one variable factor is added to it. The law of returns to scale is a long-run law: every factor is varied together, in the same proportion.

The three stages of variable proportions
Stage	Marginal product	Average product	Total product
I — increasing returns	Rises, then begins to fall	Rises throughout	Rises at an increasing rate
II — diminishing returns	Falls, reaching zero at the end	Falls after its maximum	Rises at a decreasing rate to its maximum
III — negative returns	Negative	Falls	Falls

⚠️ A rational firm produces in Stage II

Stage I is not where output is maximised but it is not rational to stop there either, because the fixed factor is being underused and adding more of the variable factor still raises average product. Stage III is irrational because output actually falls. So production settles in Stage II, and the exact point inside it is decided by prices, not by the production function.

Returns to scale are increasing when output rises more than proportionately to the equal increase in all factors, constant when it rises in the same proportion, and decreasing when it rises less. Increasing returns come from indivisibilities and specialisation; decreasing returns come from the growing difficulty of coordinating a large organisation, which is a managerial limit rather than a technical one.

Key takeaways

Extension and contraction are movements along a demand curve caused by the good's own price; increase and decrease are shifts caused by anything else. Questions mark on that exact wording.
On a straight-line demand curve elasticity is unity at the midpoint, greater above and smaller below — one curve carries every elasticity, so 'elastic demand curve' describes a region, not a shape.
MR = AR(1 − 1/e) gives three results at once: MR is zero at unit elasticity and total revenue peaks there, and a profit-maximising firm never operates where demand is inelastic.
Indifference curves cannot intersect, and the reason is transitivity of preference rather than a convention of drawing them.
Variable proportions is short run with one factor fixed; returns to scale is long run with all factors varied together. A rational firm produces in Stage II of the first.

Practice questions (9)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

A rise in consumer income raises the quantity of a normal good bought at every price. This is correctly described as:
1. An increase in demand
2. An extension of demand
3. A contraction of demand
4. A movement along the demand curve
Show answer
Answer: A — An increase in demand
The four terms are used precisely in this unit. Extension and contraction are movements along a fixed demand curve, and only a change in the good's own price can cause them. Increase and decrease are shifts of the whole curve, caused by any determinant other than the good's own price — income, the price of a related good, tastes, expectations or the number of buyers. Income has risen and the good's price has not, so the curve shifts rightward and the correct term is an increase in demand.
At a point on a firm's demand curve the price elasticity of demand has a magnitude of 2 and the price is ₹60. Marginal revenue at that point is:
1. ₹30
2. ₹120
3. ₹20
4. ₹90
Show answer
Answer: A — ₹30
Apply MR = AR(1 − 1/e). For a single-price seller average revenue is the price, so AR is ₹60, and with e equal to 2 the bracket is 1 − 0.5 = 0.5, giving MR = ₹30. The value ₹120 comes from multiplying by e instead of applying the formula, and ₹20 from using 1/3 in place of 1/2, which is the slip made by treating e as 3. ₹90 results from adding rather than subtracting inside the bracket, which would put marginal revenue above average revenue — impossible for a downward-sloping demand curve.
The cross elasticity of demand between two goods is found to be negative. The goods are:
1. Complements
2. Substitutes
3. Unrelated
4. Both inferior goods
Show answer
Answer: A — Complements
Cross elasticity measures how the demand for one good responds to the price of another, and its sign identifies the relationship. A negative value means a rise in the price of one reduces demand for the other, which is what happens when the two are consumed together — cars and petrol, printers and cartridges. A positive value marks substitutes, where a price rise in one pushes buyers towards the other. A value of zero marks unrelated goods. Inferiority is a property revealed by income elasticity, not cross elasticity, so the last option is measuring the wrong thing.
A profit-maximising monopolist will never set output in the range where demand is:
1. Inelastic
2. Elastic
3. Unit elastic
4. Perfectly elastic
Show answer
Answer: A — Inelastic
Where demand is inelastic the magnitude of elasticity is below one, so the bracket in MR = AR(1 − 1/e) is negative and marginal revenue is negative. A firm in that region could reduce output, raise total revenue and cut total cost at the same time, so it cannot be maximising profit. Profit maximisation therefore requires an output in the elastic range, where marginal revenue is positive and can be equated to a positive marginal cost. Unit elasticity is the boundary where marginal revenue is exactly zero, and perfect elasticity belongs to perfect competition rather than monopoly.
Two indifference curves of the same consumer cannot intersect because intersection would violate:
1. The transitivity of preferences
2. The law of demand
3. The budget constraint
4. The law of diminishing returns
Show answer
Answer: A — The transitivity of preferences
Suppose two curves crossed. The point of intersection lies on both, so it is indifferent to a point on the lower curve and also to a point on the higher one. Transitivity would then make those two points indifferent to each other, which is impossible because the point on the higher curve offers more of both goods and is strictly preferred. The contradiction comes from the consistency assumption about preferences and has nothing to do with the budget, the law of demand or the production function.
The law of variable proportions applies to a situation in which:
1. At least one factor is fixed and a variable factor is added to it
2. All factors are increased in the same proportion
3. All factors are fixed and only technology changes
4. Output is fixed and the factor mix is chosen to minimise cost
Show answer
Answer: A — At least one factor is fixed and a variable factor is added to it
The law of variable proportions is a short-run proposition, and the short run is defined precisely by at least one factor being fixed. Adding successive units of a variable factor to that fixed factor first raises and then reduces the marginal product, producing the familiar three stages. Increasing every factor in the same proportion is the long-run case and is described by returns to scale instead. Choosing a least-cost factor mix for a given output is the producer's equilibrium problem, solved with isoquants and isocosts, not by this law.
Under perfect competition the marginal revenue curve of an individual firm:
1. Coincides with its average revenue curve and is horizontal
2. Lies below its average revenue curve and slopes downward
3. Lies above its average revenue curve
4. Is vertical at the equilibrium output
Show answer
Answer: A — Coincides with its average revenue curve and is horizontal
A perfectly competitive firm faces a perfectly elastic demand curve, so the magnitude of elasticity is infinite. In MR = AR(1 − 1/e) the term 1/e goes to zero, leaving MR equal to AR, and both equal the market price at which the firm can sell any quantity. The two curves are therefore the same horizontal line. Marginal revenue lies below average revenue only when the seller must lower the price on all units to sell one more, which is the monopoly and monopolistic competition case, and it can never lie above average revenue for a downward-sloping demand curve.
Baumol's model of the firm holds that managers maximise:
1. Sales revenue, subject to a minimum profit constraint
2. Profit, without any constraint
3. Staff and discretionary expenditure
4. The rate of growth of assets
Show answer
Answer: A — Sales revenue, subject to a minimum profit constraint
Baumol argued that in a firm where ownership is separated from control, a manager's salary, status and security track turnover more closely than profit, so the operating objective becomes sales revenue maximisation — but subject to earning at least enough profit to keep shareholders and lenders satisfied. Unconstrained profit maximisation is the neoclassical benchmark the model departs from. Maximising staff and discretionary spending is Williamson's managerial utility model, and growth maximisation is associated with Marris; each is a distinct alternative and they are routinely tested against one another.
On a straight-line demand curve, the price elasticity of demand at the midpoint is:
1. Equal to one
2. Equal to zero
3. Infinite
4. The same as at every other point on the line
Show answer
Answer: A — Equal to one
By the point method, elasticity at any point on a straight-line demand curve equals the length of the lower segment divided by the length of the upper segment. At the midpoint the two segments are equal, so the ratio is one. Above the midpoint the lower segment is longer and elasticity exceeds one; below it the lower segment is shorter and elasticity is less than one. Elasticity is zero only where the curve meets the quantity axis and infinite only where it meets the price axis, and it emphatically is not constant along a straight line — that is the common misconception this question targets.