📖 经济学模块终测（15 题）

Economics

I. Lesson Focus

This final review lesson consolidates all major topics from the Economics curriculum: demand and supply analysis with elasticities, the four market structures and their long-run equilibria, aggregate demand–aggregate supply model, key macroeconomic indicators, monetary and fiscal policy transmission, Phillips curve relationships in short and long run, and international trade plus exchange-rate determination using purchasing-power parity and interest-rate parity. The focus is on precise formula application, numerical calculation accuracy, and avoiding common conceptual traps that appear in the actual CFA Level I exam.

II. The Problem

Economics represents 14–17% of the Level I exam. Candidates frequently lose marks on elasticity calculations using the midpoint method, distinguishing short-run versus long-run Phillips curve implications, identifying deadweight loss in trade barriers, applying both PPP and IRP simultaneously to forecast exchange rates, and understanding monetary neutrality in the long run. This lesson revisits all core definitions, mechanisms, formulas, and relationships with three detailed worked cases, a targeted trap table, and eight rigorous practice questions that mirror actual exam difficulty.

III. Core Review of Demand and Supply Analysis

Market demand curves slope downward; supply curves slope upward. Equilibrium price and quantity occur at their intersection. Price elasticity of demand is $E_d = \frac{\% \Delta Q_d}{\% \Delta P}$. The midpoint formula used on the exam is
$$E_d = \frac{(Q_2-Q_1)/((Q_2+Q_1)/2)}{(P_2-P_1)/((P_2+P_1)/2)}.$$
Absolute value > 1 indicates elastic demand, < 1 inelastic, = 1 unit elastic. Income elasticity and cross-price elasticity are also tested. Total revenue moves in the same direction as price when demand is inelastic and in the opposite direction when demand is elastic.

IV. Market Structures Comparison

Perfectly competitive firms in long-run equilibrium produce where P = MR = MC = minimum ATC, earning zero economic profit and achieving allocative and productive efficiency. Monopolists set MR = MC but charge P > MC, creating deadweight loss. Monopolistic competition reaches zero economic profit but with excess capacity. Oligopoly outcomes depend on the model (Cournot, Bertrand, etc.).

V. Macroeconomic Indicators and AD–AS Model

GDP by expenditure approach: $Y = C + I + G + (X - M)$. Natural unemployment includes frictional and structural components. Short-run aggregate supply slopes upward due to sticky wages and misperceptions; long-run AS is vertical at potential GDP. The short-run Phillips curve is downward-sloping (trade-off between inflation and unemployment); the long-run Phillips curve is vertical at the natural rate of unemployment, implying no permanent trade-off.

VI. Monetary and Fiscal Policy

Central-bank tools include open-market operations, discount rate, and reserve requirements. Expansionary monetary policy lowers interest rates, raising investment and consumption through the interest, credit, asset-price, and exchange-rate channels. The Taylor rule provides a policy-rate guideline:
$$i = r^ + \pi + 0.5(\pi - \pi^) + 0.5(y - y^*).$$
In the long run, money is neutral: monetary policy affects only nominal variables (price level, nominal wages) but not real output or unemployment.

VII. International Trade and Exchange Rates

Comparative advantage, not absolute advantage, drives mutually beneficial trade. Tariffs and quotas generate net welfare losses (deadweight loss triangles). Relative PPP states that the expected percentage change in the spot exchange rate equals the inflation differential:
$$\% \Delta S \approx \pi_d - \pi_f.$$
Interest-rate parity links forward and spot rates:
$$F/P = (1 + i_d)/(1 + i_f).$$
When real rates are equal across countries, PPP and IRP forecasts are consistent: the high-inflation currency is expected to depreciate.

Worked Cases

Case 1: Price Elasticity and Total Revenue

A good currently sells for $20 with quantity demanded of 1,000 units (total revenue = $20,000). Price falls to $18 and quantity rises to 1,150 units. Calculate price elasticity using the midpoint method and determine the effect on total revenue.

Solution:
$\% \Delta Q = (1,150 - 1,000)/((1,150 + 1,000)/2) = 150/1,075 ≈ 0.1395$ (13.95%)
$\% \Delta P = (18 - 20)/((18 + 20)/2) = -2/19 ≈ -0.1053$ (-10.53%)
$E_d = 0.1395 / 0.1053 ≈ 1.325 > 1$ → elastic.
New total revenue = 18 × 1,150 = $20,700, an increase of $700. When demand is elastic, a price decrease raises total revenue.

Case 2: Long-Run Equilibrium – Perfect Competition vs Monopoly

In a perfectly competitive industry, long-run equilibrium price is $15, output 100 units, ATC = $15. The industry becomes a monopoly; at the profit-maximizing output, MR = MC = $10 and the corresponding price on the demand curve is $25. Compare efficiency and profit outcomes.

Solution:
Perfect competition: P = MR = MC = minimum ATC = $15, zero economic profit, no deadweight loss, maximum consumer surplus and productive efficiency.
Monopoly: P = $25 > MC = $10, positive economic profit (or zero depending on ATC), lower output, positive deadweight loss, allocative and productive inefficiency.

Case 3: Combined PPP and IRP Application

U.S. inflation 2%, Japan 1%; U.S. nominal interest rate 3%, Japan 1%. Current spot rate USD/JPY = 110. Assuming PPP and IRP both hold, calculate the expected future spot rate in one year and the one-year forward rate.

Solution:
Relative PPP: expected %ΔS ≈ inflation differential = 2% – 1% = +1%.
Expected future spot ≈ 110 × 1.01 = 111.10 (yen appreciates).
IRP: $F = 110 × (1.03)/(1.01) ≈ 112.18$.
The forward rate is higher than the spot (dollar at forward discount), consistent with the interest-rate differential.

Traps

Key Formulas

• Midpoint price elasticity: $E_d = \frac{(Q_2-Q_1)/((Q_2+Q_1)/2)}{(P_2-P_1)/((P_2+P_1)/2)}$
• Total revenue–elasticity rule: if $|E_d|>1$, P↓ → TR↑; if $|E_d|<1$, P↓ → TR↓
• GDP expenditure: $Y = C + I + G + (X - M)$
• Taylor rule: $i = r^ + \pi + 0.5(\pi - \pi^) + 0.5(y - y^*)$
• Relative PPP: $\% \Delta S \approx \pi_d - \pi_f$
• Interest-rate parity: $F/P = (1 + i_d)/(1 + i_f)$
• Monopoly marginal revenue: $MR = P(1 - 1/|E_d|)$
• Long-run Phillips curve: vertical at natural rate of unemployment

Practice Questions

Q1. Using the midpoint method, if price falls from $10 to $8 and quantity demanded rises from 200 to 260 units, the price elasticity of demand is closest to:
A. 0.6
B. 1.2
C. 1.5
D. 2.0

Q2. In long-run equilibrium of a perfectly competitive market, which condition must hold?
A. P > MC
B. P = MR = MC = minimum ATC
C. Positive economic profit exists
D. Firms face downward-sloping demand curves

Q3. Which of the following shifts the short-run aggregate supply curve to the left?
A. Technological improvement
B. Increase in labor costs
C. Government tax cut
D. Rise in consumer confidence

Q4. According to the Taylor rule, if the target inflation rate is 2%, actual inflation is 5%, equilibrium real rate is 2%, and output gap is +3%, the policy rate should be closest to:
A. 2%
B. 5%
C. 8.5%
D. 10.5%

Q5. If real interest rates are equal across two countries, according to international Fisher effect and PPP, the currency of the higher-inflation country is expected to:
A. Appreciate
B. Depreciate
C. Remain unchanged
D. Cannot be determined

Q6. After imposition of an import tariff, which statement is correct?
A. Consumer surplus increases
B. Producer surplus decreases
C. Deadweight loss exists
D. Government receives no revenue

Q7. If a central bank purchases government bonds in open-market operations, the most likely immediate effect is:
A. Higher interest rates
B. Decreased money supply
C. Increased aggregate demand
D. Lower inflation rate

Q8. In the long run, monetary policy primarily affects:
A. Real output
B. Unemployment rate
C. Nominal variables such as the price level
D. Real interest rates

Answers

Takeaways

• Always apply the midpoint formula for elasticity and link elasticity > 1 to rising total revenue when price falls.
• Perfect competition achieves efficiency with zero long-run economic profit; monopoly creates deadweight loss.
• Short-run Phillips curve shows inflation–unemployment trade-off; long-run curve is vertical at natural unemployment.
• Monetary policy influences real variables in the short run but only nominal variables in the long run.
• PPP forecasts exchange-rate change by inflation differential; IRP determines forward premium/discount by nominal interest differential.
• Trade barriers (tariffs, quotas) generate net welfare losses; comparative advantage remains the foundation of beneficial trade.